The Collaborative International Dictionary of English v.0.48:

Peccadillo \Pec`ca*dil"lo\, n.; pl. Peccadillos. [Sp.
   pecadillo, dim. of pecado a sin, fr. L. peccatum. See
   Peccant.]
   A slight trespass or offense; a petty crime; a trifling
   fault. --Sir W. Scott.
   [1913 Webster]

WordNet (r) 3.0 (2006):

peccadillo
    n 1: a petty misdeed [syn: indiscretion, peccadillo]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "peccadillo":
   atrocity, blunder, botch, breach, crime, crime against humanity,
   deadly sin, delinquency, dereliction, enormity, error, evil,
   failure, fault, faux pas, felony, gaffe, genocide, goof,
   guilty act, heavy sin, impropriety, indiscretion, inexpiable sin,
   infraction, iniquity, injury, injustice, lapse, malefaction,
   malfeasance, malum, minor wrong, misdeed, misdemeanor, misfeasance,
   misstep, mistake, mortal sin, nonfeasance, offense, omission,
   outrage, peccancy, shortcoming, sin, sin of commission,
   sin of omission, sinful act, slip, stumble, tort, transgression,
   trespass, trip, unutterable sin, venial sin, violation, wrong