The Collaborative International Dictionary of English v.0.48:

Unquenchable \Unquenchable\
   See quenchable.

WordNet (r) 3.0 (2006):

unquenchable
    adj 1: impossible to quench; "unquenchable thirst" [syn:
           quenchless, unquenchable]

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "unquenchable":
   a hog for, acquisitive, all-devouring, avaricious, avid,
   bottomless, coveting, covetous, deathless, devouring, esurient,
   gluttonous, gobbling, grabby, grasping, greedy, hoggish, immortal,
   imperishable, impregnable, incorruptible, indelible,
   indestructible, ineffaceable, ineradicable, inexpugnable,
   inextinguishable, insatiable, insatiate, invincible, invulnerable,
   irrepressible, limitless, mercenary, miserly, money-hungry,
   money-mad, nonperishable, omnivorous, overgreedy, piggish,
   quenchless, rapacious, ravening, ravenous, slakeless, sordid,
   swinish, unappeasable, unappeased, undampable, undestroyable,
   undying, unsated, unsatisfiable, unsatisfied, unslakeable,
   unslaked, venal, voracious