Green’s Dictionary of Slang

an assignation; also as v.

c.1698		B.E. Dict. Canting Crew n.p.: Assig, now us’d for Assignation, an Appointment or meeting.
1725		New Canting Dict.
1737		Bailey Universal Etym. Eng. Dict.
1785		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1821		R. Waln Hermit in America on Visit to Phila. 2nd series 26: Had an assig. with a Quicunque Vult — a snug little cinder-gabbler.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1846		Swell’s Night Guide 108/2: Assig, to meet a woman secretly.