Green’s Dictionary of Slang

1. semen, at the point of ejaculation.

1748–49		Cleland Memoirs of a Woman of Pleasure (1985) 83: His oily balsamic injection mixing deliciously with the sluices in flow from me.
c.1900		‘Ramrod’ Nocturnal Meeting 91: A copious injection into the two plump sterns.

2. sexual intercourse; the intromission of the penis.

1843		Gleaner (Manchester, NH) 20 May n.p.: Had’nt you better go to Captain Smoothee an[d] take an injection.
1888–94		‘Walter’ My Secret Life (1966) VI 1256: Frigged her now and then till we could both bear delay no longer, and then gave her another injection.