Lackin is Chicago's version of the slang term slippin. An individual is lacking when they're no This word became popular as Chicago’s drill music started becoming popular following Chief Keef’s rise to fame for his song “I Don’t Like”, which featured GBE member ‘Lil Reese’.
If someone's walking through rival neighborhood without any form of weapon, they're lackin. If someone's not paying attention to their surroundings when they have a target on their back, they're lackin.
In Lil Herb’s ‘Kill Sh*t’ song featuring Lil Bibby, Bibby says “Catch me lackin'? I doubt that, cause I never leave without that - And that's real sh*t, got real hittas they kill sh*t”. Bibby is saying he doubts you’ll ever catch him unready (lackin) for a shootout.