petite amie
/pəˌtiːt aˈmiː/ /pəˌtiːt əˈmiː/noun
The (usually young) female lover of an older man; a mistress.
Origin
Late 19th century; earliest use found in Ernest Dowson (1867–1900), poet. Apparently from French petite amie from petite, feminine of petit + amie, feminine form corresponding to ami friend.