Green’s Dictionary of Slang

(UK black) a knife.

1997		C. Newland Scholar 34: Dem man had their boras y’know.
2017		Eve. Standard 4 July 8/4: ‘You want to know why young people carry borra these days?’ said Wayne, referring to knives .
2020		G. Krauze What They Was 46: I imagine myself running into her yard on a rago tip, bareface with a borer, and start shanking her stepdad.