The Collaborative International Dictionary of English v.0.48:

Untrustworthy \Untrustworthy\
   See trustworthy.

WordNet (r) 3.0 (2006):

untrustworthy
    adj 1: not worthy of trust or belief; "an untrustworthy person"
           [syn: untrustworthy, untrusty] [ant: trustworthy,
           trusty]

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "untrustworthy":
   dangerous, desultory, doubtful, dubious, fly-by-night, hazardous,
   infirm, insecure, insubstantial, irresponsible, perilous,
   precarious, provisional, questionable, risky, rocky, shaky,
   shifting, shifty, slippery, temporary, tentative, ticklish,
   tottery, treacherous, trustless, unassured, uncertain,
   undependable, unfaithworthy, unhealthy, unpredictable, unreliable,
   unsafe, unsolid, unsound, unstable, unsteadfast, unsteady,
   unsubstantial, unsure, untrusty