The Collaborative International Dictionary of English v.0.48:

Idiosyncratic \Id`i*o*syn*crat"ic\, Idiosyncratical
\Id`i*o*syn*crat"ic*al\, a.
   Of peculiar temper or disposition; belonging to one's
   peculiar and individual character.
   [1913 Webster]

WordNet (r) 3.0 (2006):

idiosyncratic
    adj 1: peculiar to the individual; "we all have our own
           idiosyncratic gestures"; "Michelangelo's highly
           idiosyncratic style of painting"

Moby Thesaurus II by Grady Ward, 1.0:

102 Moby Thesaurus words for "idiosyncratic":
   abnormal, anomalous, appropriate, characteristic, characterizing,
   connotative, contrastive, crank, crankish, cranky, crotchety,
   demonstrative, denominative, denotative, designative, deviant,
   deviative, diacritical, diagnostic, differencing, different,
   differential, differentiative, discriminating, discriminative,
   distinctive, distinguished, distinguishing, divergent, dotty,
   eccentric, emblematic, erratic, evidential, exceptional,
   exhibitive, expressive, fey, figural, figurative, flaky, freakish,
   funny, identifying, ideographic, idiocratic, implicative,
   in character, indicating, indicative, indicatory, individual,
   individualizing, individuating, intrinsic, irregular, kinky, kooky,
   maggoty, marked, meaningful, metaphorical, naming, nutty, odd,
   oddball, pathognomonic, peculiar, personal, personalizing, private,
   proper, queer, quintessential, quirky, representative, screwball,
   screwy, semantic, semiotic, separative, signalizing, significant,
   significative, signifying, single, singular, strange, suggestive,
   symbolic, symbolistic, symbological, symptomatic, symptomatologic,
   true to form, twisted, typical, unconventional, unique, unnatural,
   wacky, whimsical