The Collaborative International Dictionary of English v.0.48:

Lunacy \Lu"na*cy\, n.; pl. Lunacies. [See Lunatic.]
   1. Insanity or madness; properly, the kind of insanity which
      is broken by intervals of reason, -- formerly supposed to
      be influenced by the changes of the moon; any form of
      unsoundness of mind, except idiocy; mental derangement or
      alienation. --Brande. --Burrill.
      [1913 Webster]

            Your kindred shuns your house
            As beaten hence by your strange lunacy. --Shak.
      [1913 Webster]

   2. A morbid suspension of good sense or judgment, as through
      fanaticism. --Dr. H. More.

   Syn: Derangement; craziness; mania. See Insanity.
        [1913 Webster]

WordNet (r) 3.0 (2006):

lunacy
    n 1: obsolete terms for legal insanity [syn: lunacy,
         madness, insaneness]
    2: foolish or senseless behavior [syn: folly, foolery,
       tomfoolery, craziness, lunacy, indulgence]

Moby Thesaurus II by Grady Ward, 1.0:

97 Moby Thesaurus words for "lunacy":
   aberration, abnormality, absurdity, alienation, asininity,
   battiness, brain damage, brainlessness, brainsickness, buffoonery,
   clouded mind, clownishness, crackpottedness, crankiness, craziness,
   daffiness, daftness, dementedness, dementia, derangement,
   desipience, disorientation, distraction, eccentricity, fatuity,
   fatuousness, folie, folly, foolery, foolhardiness, foolheadedness,
   foolishness, frivolity, frivolousness, furor, giddiness, goofiness,
   idiocy, illogic, illogicality, imbecility, inanity, ineptitude,
   insaneness, insanity, irrationality, loss of mind, loss of reason,
   madness, mania, mental deficiency, mental derangement,
   mental disease, mental disorder, mental disturbance,
   mental illness, mental instability, mental sickness,
   mind overthrown, mindlessness, mindsickness, niaiserie, nugacity,
   nuttiness, oddness, pixilation, possession, preposterousness,
   psychopathy, psychosis, queerness, rabidness, reasonlessness,
   ridiculousness, sappiness, screwiness, senselessness,
   shattered mind, sick mind, sickness, silliness, strangeness,
   stupidity, thoughtlessness, triflingness, triviality, unbalance,
   unbalanced mind, unsaneness, unsound mind, unsoundness,
   unsoundness of mind, wackiness, weirdness, witlessness, zaniness,
   zanyism



Bouvier's Law Dictionary, Revised 6th Ed (1856):

LUNACY, med. jur. A disease of the mind, which is differently defined as it
applies to a class of disorders, or only to one species of them. As a
general term it includes all the varieties of mental, disorders, not
fatuous.
     2. Lunacy is adopted as a general term, on account of its general use
as such in various legislative acts and legal proceedings, as commissions of
lunacy, and in this sense it seems to be synonymous with non compos mentis,
or of unsound mind.
     3. In a more restricted sense, lunacy is the state of one who has bad
understanding, but by disease, grief, or other accident, has lost the use of
reason. 1 Bl. Com. 304.
     4. The following extract from a late work, Stock on the Law of Non
Compotes Mentis, will show the difficulties of discovering what is and what
is not lunacy. "If it be difficult to find an appropriate definition or
comprehensive name for the various species of lunacy," says this author,
page 9, "it is quite as difficult to find anything approximating to a
positive evidence of its presence. There are not in lunacy, as in fatuity,
external signs not to be mistaken, neither is there that similarity of
manner and conduct which enables any one, who has observed instances of
idiocy or imbecility, to detect their presence in all subsequent cases, by
the feebleness of perception and dullness of sensibility common to them all.
The varieties of lunacy are as numerous as the varieties of human nature,
its excesses commensurate with the force of human passion, its phantasies
coextensive with the range of human intellect. It may exhibit every mood
from the most serious to the most gay, and take every tone from the most
sublime to the most ridiculous. It may confine itself to any trifling
feeling or opinion, or overcast the whole moral and mental conformation. It
may surround its victim with unreal persons and events, or merely cause him
to regard real persons and events with an irrational favor or dislike,
admiration or contempt. It may find satisfaction in the most innocent folly,
or draw delight from the most atrocious crime. It may lurk so deeply as to
elude the keenest search, or obtrude so openly as to attract the most
careless notice. It may be the fancy of an hour, or the distraction of a
whole life. Such being the fact, it is not surprising that many scientific
and philosophical men have vainly exhausted their observation and ingenuity
to find out some special quality, some peculiar mark or characteristic
common to all cases of lunacy, which might serve at least as a guide in
deciding on its absence or presence in individual instances. Being hopeless
of a definition, they would willingly have contented themselves with a test,
but even this the obscurity and difficulty of the subject seem to forbid.
     5. Lord Erskine, who, in his practice at the bar, had his attention
drawn this way, from being engaged in some of the most remarkable trials of
his time involving questions of lunacy, has given as his test, "a delusive
image, the inseparable companion of real insanity," (Ersk. Misc. Speeches)
and Dr. Haslam, whose opportunities of observation have surpassed most other
persons, has proposed nearly the same, by saying that "false belief is the
essence of insanity." (Haslam on Insanity.) Sir John Nicholl, in his
admirable judgment in the case of Dew v. Clark, thus expresses himself: "The
true criterion is, where there is delusion of mind there is insanity; that
is, when persons believe things to exist, which exist only, or at least, in
that degree exist only in their own imagination, and of the non-existence of
which neither argument nor proof can convince them; they are of unsound mind;

or as one of the counsel accurately expressed it, it is only the belief of
facts, which no rational person could have believed, that is insane
delusion." (Report by Haggard, p. 7.) Useful as these several remarks are,
they are not absolutely true. It is indeed beyond all question that the
great majority of lunatics indulge in some "delusive image," entertain some
"false belief." They assume the existence of things or persons which do not
exist, and so yield to a delusive image, or they come to wrong conclusions
about persons and things which do exist, and so fall into a false belief.
But there is a class of cases where lunacy is the result of exclusive
indulgence in particular trains of thought or feeling, where these tests are
sometimes wholly wanting, and yet where the entire absorption of the
faculties in one predominant idea, the devotion of all the bodily and mental
powers to one useless or injurious purpose, prove that the mind has lost its
equilibrium. With some passions, indeed, such as self-esteem and fear, what
was at first an engrossing sentiment, will often go on to a positive
delusion; the self-adoring egotist grows to fancy himself a sovereign or a
deity; the timid valetudinarian becomes the prey of imaginary diseases, the
victim of unreal persecutions. But with many other passions, such as desire,
avarice or revenge, the neglect and forgetfulness of all things save one,
the insensibility to all restraints of reason, morality, or prudence, often
proceed to such an extent as to justify holding an individual as a lunatic,
incapable of all self-restraint, although, strictly speaking, not possessed
by any delusive image or false belief. Much less do these tests apply to
many cases of irresistible propensity to acts wholly irrational, such as to
murder or to steal without the smallest assignable motive, which, rare as
they are, certainly occur from time to time, and cannot but be held as an
example of at least partial and temporary lunacy. It is to cases where no
false belief or image can be detected, that the remark of Lord Erskine is
more particularly applicable; "they frequently mock the wisdom of the wisest
in judicial trials," (Ersk. Misc. Speeches,) and were not the paramount
object of all legal punishment the benefit of the community, which makes it
inexpedient to spare offenders against the law, if insanity be the ground of
their defence, except upon the clearest proof, lest skillful dissemblers
should thereby be led to hope for impunity, very subtle questions might no
doubt be raised as to the degree of moral responsibility and mental sanity
attaching to the perpetrators of many atrocious acts, seeing that they often
commit them tinder temptations quite inadequate to allure men of common
prudence, or under passions so violent as to suspend altogether the
operations of reason or free will. For as it is impossible to obtain an
accurate definition of lunacy, so it is manifestly so, to draw the line
correctly between it and its opposite rationality, or, to borrow the words
of Chief Justice Hale, (1 Hale's P. C. p. 30,) "Doubtless most persons that
are felons, of themselves and others, are under a degree of partial insanity
when they commit those offences. It is very difficult to define the
indivisible line that divides perfect and partial, insanity; but it must
rest on circumstances duly to be weighed and considered both by the judge
and jury, lest on one side there be a kind of inhumanity towards the defects
of human nature, or on the other side too great an indulgence given to great
crimes."