WordNet (r) 3.0 (2006):

undependable
    adj 1: not worthy of reliance or trust; "in the early 1950s
           computers were large and expensive and unreliable"; "an
           undependable assistant" [syn: unreliable,
           undependable] [ant: dependable, reliable]
    2: liable to be erroneous or misleading; "an undependable
       generalization" [syn: undependable, unreliable]

Moby Thesaurus II by Grady Ward, 1.0:

109 Moby Thesaurus words for "undependable":
   adrift, afloat, alternating, amorphous, capricious, changeable,
   changeful, coquettish, dangerous, desultory, deviable, dizzy,
   doubtful, dubious, eccentric, erratic, faddish, fast and loose,
   fickle, fitful, flickering, flighty, flirtatious, flitting,
   fluctuating, fly-by-night, freakish, giddy, hazardous, impetuous,
   impulsive, inconsistent, inconstant, indecisive, infirm, insecure,
   insubstantial, irregular, irresolute, irresponsible, light, mazy,
   mercurial, moody, perilous, precarious, provisional, quicksilver,
   rambling, restless, risky, rocky, roving, scatterbrained, shaky,
   shapeless, shifting, shifty, shuffling, skittish, slippery,
   spasmodic, spineless, temporary, tentative, ticklish, tottery,
   toying, treacherous, tricky, trustless, unaccountable, uncertain,
   uncontrolled, undisciplined, unfaithworthy, unfixed, unhealthy,
   unpredictable, unreliable, unrestrained, unsafe, unsettled,
   unsolid, unsound, unstable, unstable as water, unstaid,
   unsteadfast, unsteady, unsubstantial, unsure, untrustworthy,
   untrusty, vacillating, vagrant, variable, versatile,
   vicissitudinary, vicissitudinous, volatile, wandering, wanton,
   wavering, wavery, wavy, wayward, whimsical, wishy-washy