Now stay in line, don't step on cracks
So you break her back I'm talking 'bout your mom

from A$AP Rocky – Long.Live.A$AP Lyrics on Rap Genius

Meaning

A play on the kids rhyme “step on a crack, break your mother’s back”. In this instance Rocky is advising you (or himself) to stay clean and straight so your mama doesn’t end up worrying about you.

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