Green’s Dictionary of Slang

(orig. UK black) to smoke cannabis or crack cocaine.

1997		C. Newland Scholar 172: ‘Buil’ a spliff man,’ [...] ‘I don’t smoke.’ [...] ‘Someone who can blas’ a Rad car like dat, must bun.’.
2014		Wiley ‘Grew Up In’ 🎵 I don’t like that yout, I don’t care who's bunning.
2020		G. Krauze What They Was 21: Bun cro till you zone out .