What is "indeclinable"

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.

late 14c., originally in grammar, from French indéclinable, from Latin indeclinabilis, from indeclinatus "unchanged, constant," from in- "not, opposite of" (see in- (1)) + declinatus, from declinare (see decline (v.)). Related: Indeclinably.

a. (context grammar of a word English) not grammatically inflected, especially if others of its class are usually inflected n. (context grammar English) A word that is not grammatically inflected.