The definition of indeclinable is a grammar term that is a word that must be determined from its context because it does not have anything added to show a grammatical category such as tense, gender, mood or number.
(adjective)An example of an indeclinable word is the word "must" or the word "coffee."
See indeclinable in Webster's New World College Dictionary
adjective
See indeclinable in American Heritage Dictionary 4
adjective