Green’s Dictionary of Slang

(US) a mistress, a kept woman.

1931–4		D. Clemmer Prison Community (1940) 331/1: keptee, n. A kept woman.
1949		Monteleone Criminal Sl. (rev. edn).
1952		Lait & Mortimer USA Confidential 250: Hot shots dine their expensive kepties at Lavisi’s.
1966		Trimble 5000 Adult Sex Words and Phrases.
1972		R.A. Wilson Playboy’s Book of Forbidden Words.
1989		(con. 1930s) Courtwright & Des Jarlais Addicts Who Survived 171: At the very top of the heap were the ‘kepties’ – mistresses of extraordinary beauty or charm who were supported by wealthy men.