fill in, v.² — Green’s Dictionary of Slang

fill in v.²

[the image is of ‘filling’ one’s victim’s face with a fist, to give them a black eye]

(orig. naut.) to beat up.

1959		C. MacInnes Absolute Beginners 81: When the cowboys start to fill me in, I’ll have you buzzed immediately for bail.
1965		L. Dunne Goodbye to The Hill (1966) 101: Her two brothers started filling me in.
1979		F. Norman Too Many Crooks Spoil the Caper 133: Where is he, the sod? I’ll fill him in, I’ll fill him in!
1982		G. Newbold Big Huey 133: If he hadn’t been so small, he told him, they’d have filled him in right then and there.
1993		I. Welsh Trainspotting 111: Dawsy goat filled in up thair a wee bit back.
2002		J. Cameron Hell on Hoe Street 38: Nicky you try to fill him in and he probably hit you with his sword.