Green’s Dictionary of Slang

dark v.

1. (UK Und.) to knock out.

[UK]Swell’s Night Guide 57: gonniff: Faked it rumbo: copped the lob, darked the hommo of the cassey, and scarpered with the swag, bona.

2. (UK Und.) to darken, to extinguish the light.

[UK]Swell’s Night Guide 58: I take my green’un; pinches his skin and ticker, darks the lumber, and planted Flabby Bet on him.