Green’s Dictionary of Slang

1. back-up, usu. in the form of physical power.

1988		S. Selvon Eldorado West One 108: Delighted to be of assist, old chap.
1995		J. Cameron Vinnie Got Blown Away 65: Fact is needs a geezer bigger than me. Personal matter only could be big readies in it. I need your assist.
2002		J. Cameron Hell on Hoe Street 177: He wanted my assist lifting some geezer shafted a copper.

2. the person drawn in to provide this back-up.

1995

J. Cameron Vinnie Got Blown Away 61: One place I couldn’t go for an assist, or not unless the odds improved.