Green’s Dictionary of Slang

injection n.

[see Williams (1994) for metaphorical use in 17C]

1. semen, at the point of ejaculation.

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

2. sexual intercourse; the intromission of the penis.

[UK]‘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.