Green’s Dictionary of Slang

the bastard child of a woman who has been seduced and abandoned by a married man.

a.1821		H. Walpole in Indicator n.p.: I love David too well not to be jealous of an Abishag eight years old.
1875		Western Dly Press 7 June 3/4: A reverend proprietor of racehorses [...] celebrated a marriage at a green old age with a lady who formerly served him in a menial capacity [...] He has not only left his fortune [...] but beuqeathed her a successor in matrimonial honours [...] planning a future marriage for the Abishag who has so tenderly cherished his declining years.
1889–90		Barrère & Leland Dict. of Sl., Jargon and Cant.