The Collaborative International Dictionary of English v.0.48:

Unforeseeable \Unforeseeable\
   See foreseeable.

The Collaborative International Dictionary of English v.0.48:

Unforeseeable \Un`fore*see"a*ble\, a.
   Incapable of being foreseen. --South.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unforeseeable
    adj 1: incapable of being anticipated; "unforeseeable
           consequences"

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "unforeseeable":
   accidental, adventitious, agnostic, aleatory, ambiguous,
   capricious, casual, causeless, chance, chancy, changeable,
   contingent, destinal, dicey, doubting, equivocal, erratic,
   extraordinary, fatal, fatidic, fickle, fluky, fortuitous, hesitant,
   hesitating, iffy, improbable, incalculable, incidental, indecisive,
   indemonstrable, indeterminate, irresolute, more than expected,
   out-of-the-way, past expectation, polysemous, risky, skeptical,
   sudden, touch-and-go, unaccountable, unannounced, unanticipated,
   uncaused, uncertain, unconfirmable, unconvinced, undetermined,
   undivinable, undivined, unexpected, unforeseen, unguessed,
   unheralded, unhoped for, unlooked-for, unpersuaded, unpredictable,
   unpredicted, unprepared for, unprovable, unsure, unverifiable,
   variable, wavering, whimsical, without warning