Green’s Dictionary of Slang

1. a wife.

1615

G. Sandys Travels 66: Yet are they [Turks] to meddle with none but their owne peculiars: the offending woman they drowne, and the man they gansh [i.e. impale] [OED].

2. a mistress; occas. a male lover.

1688		T. Shadwell Squire of Alsatia IV i: I am resolved to manage my natural, my pure blowen, my, my convenient, my peculiar, my tackle, my purest pure, as the rest of the young gentlemen of the town do.
c.1698		B.E. Dict. Canting Crew n.p.: Peculiar c. a Mistress.
1753		Dict. of Love n.p.: paramour A Favourite gallant; a peculiar, a minion.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1859		Matsell Vocabulum.