Green’s Dictionary of Slang

a prostitute; also attrib.

[1781		Belle’s Stratagem 60: [stage direction] Mrs Racket, and another Impure Companion, in a Phaeton].
1784		Rambler’s Mag. Jan. 38/2: His lordship has made some tender overtures to Mrs C—r, who [...] has greedily accepted them and will accordingly figure away this winter as one of the tonish impures.
c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Impure, an Impure, a Modern term for a lady of easy virtue.
1788		Grose Classical Dict. of the Vulgar Tongue (2nd edn).
1791		R. Dighton [caption] Two Impures of the Ton driving to the Gigg Shop, Hammersmith.
1793		Sporting Mag. Dec. III 47/1: The Duke of Rutland [...] was reputed to be much attached to the noted courtesan Peg Plunket [...] in the box above him [at the theatre] sat his impure favourite.
1796		Sporting Mag. Apr. VIII 40/2: A young haberdasher from Fleet-street, with his impure from Portland-street Marylebone, were overset in their gig.
1802		Sporting Mag. Jan. XIX 212/2: One of his plans was to parade opposite the window of some elegant impure.
1811		Lex. Balatronicum [as cit. 1788].
1821		‘An Amateur’ Real Life in London I 67: In keeping a dashing impure, who publicly squanders away his money.
1830		Egan Finish to the Adventures of Tom and Jerry (1889) 129: A thief – and an impure.
1838		Crim.-Con. Gaz. 6 Oct. 49/1: To these he adds the society of a tonish impure, who publicaly exhausts his treasure.
1859		Matsell Vocabulum.
1896		Farmer Vocabula Amatoria (1966) 160: Impure, f. A harlot. Also in English.