No Lackin is a slang word which means that you are ready for any situation that might rise. This slang is often used by gang members to let others know they are ready for any confrontation . This slang became popular due to the rise of drill music in Chicago, where many gang members who rap, use it frequently in their songs to remind their enemies they are always prepared for battle.
Rapper Lil Mister had a song called “No Lackin” featuring P. Rico in which he says “Can't trust a soul n*ggas plotin on me - Keep them eyes open ain't no lacking homie”. Lil Mister is in someway saying that people are out to get him therefore he needs to keep his eyes open and weapon ready(no lackin).
Prior to this song, the term ‘No Lackin’ was rarely heard in rap music, but ever since rapper Chief Keef became famous and Chicago rappers started gaining momentum, numerous Chicago slangs started becoming mainstream. The opposite of no lackin is 'lackin'.