The Collaborative International Dictionary of English v.0.48:

Demency \De"men*cy\, n. [L. dementia, fr. demens mad. See
   Dement.]
   Dementia; loss of mental powers. See Insanity.
   [1913 Webster]

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

DEMENCY, dementia, med. jur. A defect, hebetude, or imbecility of the under
standing, general or partial, but confined to individual faculties of the
mind, particularly those concerned in associating and comparing ideas,
whence proceeds great, confusion and incapacity in arranging the thoughts. 1
Chit. Med. Jur. 351; Cyclop. Practical Med. tit. Insanity; Ray, Med. Jur.
ch. 9; 1 Beck's Med. Jur. 547.
     2. Demency is attended with a general enfeeblement of the moral and
intellectual faculties, consequence of age or disease, which were originally
well developed and sound. It is characterised by forgetfulness of the past;
indifference to the present and future, and a childish disposition. It
differs from idiocy and imbecility. In these latter, the powers of the mind
were never possessed, while in demency, they have been lost.
     3. Demency may also be distinguished from mania, with which it is
sometimes confounded. In the former, the mind has lost its strength, and
thereby the reasoning faculty is impaired; while in the latter, the madness
arises from an exaltation of vital power, or from a morbid excess of
activity.
     4. Demency is divided into acute and chronic. The former is a
consequence of temporary errors of regimen, fevers, hemorrhages, &c., and is
susceptible of cure the latter, or chronic demency, may succeed mania,
apoplexy, epilepsy, masturbation, and drunkenness, but is generally that
incurable decay of the mind which occurs in old age.
     5. When demency has been fully established in its last stages, the acts
of the individual of a civil nature will be void, because the party had no
consenting mind. Vide Contracts; Wills; 2 Phillim. R. 449. Having no legal
will or intention, he cannot of course commit a crime. Vide Insanity; Mania.