The Collaborative International Dictionary of English v.0.48:

Inconstant \In*con"stant\, a. [L. inconstans: cf. F. inconstant.
   See In- not, and Constant.]
   Not constant; not stable or uniform; subject to change of
   character, appearance, opinion, inclination, or purpose,
   etc.; not firm; unsteady; fickle; changeable; variable; --
   said of persons or things; as, inconstant in love or
   friendship. "The inconstant moon." --Shak.
   [1913 Webster]

         While we, inquiring phantoms of a day,
         Inconstant as the shadows we survey!     --Boyse.

   Syn: Mutable; fickle; volatile; unsteady; unstable;
        changeable; variable; wavering; fluctuating.
        [1913 Webster]

WordNet (r) 3.0 (2006):

inconstant
    adj 1: likely to change frequently often without apparent or
           cogent reason; variable; "inconstant affections"; "an
           inconstant lover"; "swear not by...the inconstant moon"-
           Shakespeare [ant: constant]

Moby Thesaurus II by Grady Ward, 1.0:

200 Moby Thesaurus words for "inconstant":
   adrift, afloat, alternating, amorphous, brittle, broken,
   capricious, careening, catchy, changeable, changeful, changing,
   choppy, coquettish, corruptible, deciduous, derelict, desultory,
   deviable, deviating, deviative, deviatory, different, disaffected,
   disconnected, discontinuous, disloyal, disorderly, disregardful,
   divaricate, divergent, diversified, diversiform, dizzy, dying,
   eccentric, elusive, ephemeral, erose, erratic, evanescent, faddish,
   fading, faithless, false, fast and loose, fickle, fitful, fleeting,
   flickering, flighty, flirtatious, flitting, fluctuating,
   fly-by-night, flying, fragile, frail, freakish, fugacious,
   fugitive, giddy, guttering, halting, herky-jerky, heteroclite,
   immethodical, impermanent, impetuous, impulsive, inattentive,
   inconsistent, indecisive, indefinite, infirm, inobservant,
   insubstantial, intermittent, intermitting, irregular, irresolute,
   irresponsible, jagged, jerky, light, lubricious, lurching, mazy,
   mercurial, momentary, moody, mortal, motley, mutable, negligent,
   nonadherent, noncompliant, nonconforming, nonconformist,
   nondurable, nonobservant, nonpermanent, nonstandard, nonuniform,
   not true to, of bad faith, passing, patchy, perishable,
   pluralistic, protean, quicksilver, ragged, rambling, recreant,
   restless, rough, roving, scatterbrained, scrappy, shapeless,
   shifting, shifty, short-lived, shuffling, skittish, snatchy,
   spasmatic, spasmic, spasmodic, spastic, spineless, sporadic,
   spotty, staggering, temperamental, temporal, temporary, ticklish,
   toying, transient, transitive, transitory, trothless,
   unaccountable, uncertain, uncompliant, unconforming, uncontrolled,
   undependable, undisciplined, undurable, unenduring, unequable,
   unequal, uneven, unfaithful, unfixed, unloyal, unmethodical,
   unmetrical, unobservant, unorthodox, unpredictable, unregular,
   unreliable, unrestrained, unrhythmical, unsettled, unstable,
   unstable as water, unstaid, unsteadfast, unsteady, unsystematic,
   untrue, ununiform, vacillating, vagrant, vague, variable,
   variegated, variform, various, varying, veering, versatile,
   vicissitudinary, vicissitudinous, volatile, wandering, wanton,
   wavering, wavery, wavy, wayward, whimsical, wishy-washy, wobbling,
   wobbly