1) French word used in English 2) Not capable of being declined 
In`de·clin'a·ble adjective [ Latin indeclinabilis : confer French indéclinable . See In- not, and Decline .] (Gram.) Not declinable; not varied by inflective terminations; as, nihil (nothing), in Latin, is an indeclinable noun. -- 
not capable of being declined; having no inflected forms: used esp. of a word belonging to a form class most of whose members are declined, as the Latin adjective decem, “ten.”