Green’s Dictionary of Slang

(UK Black) to lose concentration, to become vulnerable; to blunder.

2014		Perm ‘Dotty on the Ride’ 🎵 Catch a opp lackin, ‘woosh!’ make a nigga really hold thirty.
2015		150 ‘4 Door Truck’ 🎵 Ramesha on my hip so I don't lack.
2018-20		T. Thorne (ed.) ‘Drill Slang Glossary’ at Forensic Linguistic Databank 🌐 Lack - lose concentration, expose oneself to danger, slip up.