indeclinable

adjective

in·​de·​clin·​able ˌin-di-ˈklī-nə-bəl 

: having no grammatical inflections

Word History

Etymology

Middle English, from Late Latin indeclinabilis, from Latin in- + Late Latin declinabilis capable of being inflected, from Latin declinare to inflect — more at decline entry 1

First Known Use

14th century, in the meaning defined above