Definition of genitive case in US English:

genitive case

noun

  • (In inflected languages) a case of nouns and pronouns, and of words in grammatical agreement with them, the typical function of which is to indicate that the person or thing denoted by the word is related to another as source, possessor, or the like; (also in uninflected languages) a word or word form having a similar possessive function.

Origin

Late Middle English; earliest use found in John Trevisa (c1342–?1402), translator.