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]