The Collaborative International Dictionary of English v.0.48:

Unsteadfast \Unsteadfast\
   See steadfast.

Moby Thesaurus II by Grady Ward, 1.0:

105 Moby Thesaurus words for "unsteadfast":
   adrift, afloat, alternating, amorphous, capricious, changeable,
   changeful, dangerous, derelict, desultory, deviable, disaffected,
   disloyal, dizzy, eccentric, erratic, faithless, false,
   fast and loose, fickle, fitful, flickering, flighty, flitting,
   fluctuating, freakish, giddy, hazardous, impetuous, impulsive,
   inconsistent, inconstant, indecisive, infirm, insecure,
   insubstantial, irregular, irresolute, irresponsible, mazy,
   mercurial, mobile, moody, moving, not true to, of bad faith,
   perilous, precarious, provisional, rambling, recreant, restless,
   risky, roving, scatterbrained, shaky, shapeless, shifting, shifty,
   shuffling, slippery, spasmodic, spineless, temporary, tentative,
   ticklish, treacherous, trothless, unaccountable, uncertain,
   uncontrolled, undependable, undisciplined, unfaithful,
   unfaithworthy, unfixed, unloyal, unpredictable, unreliable,
   unrestrained, unsettled, unsolid, unsound, unstable,
   unstable as water, unstaid, unsteady, unsubstantial, unsure,
   untrue, untrustworthy, vacillating, vagrant, variable,
   vicissitudinary, vicissitudinous, volatile, wandering, wanton,
   wavering, wavery, wavy, wayward, whimsical, wishy-washy