indeclinable (adj.)

late 14c., originally in grammar, from French indéclinable or directly from Latin indeclinabilis "unchangeable," also in grammar, from indeclinatus "unchanged, constant," from in- "not, opposite of" (see in- (1)) + declinatus, from declinare "to lower; avoid, deviate; bend from, inflect" (see decline (v.)). Related: Indeclinably.