The Collaborative International Dictionary of English v.0.48:

Indeclinable \In`de*clin"a*ble\, a. [L. indeclinabilis: cf. F.
   ind['e]clinable. See In- not, and Decline.] (Gram.)
   Not declinable; not varied by inflective terminations; as,
   nihil (nothing), in Latin, is an indeclinable noun. -- n. An
   indeclinable word.
   [1913 Webster]