Lean all on the square
That's a fuckin' rhombus

from Chance The Rapper (Ft. Ab-Soul) – Smoke Again Lyrics on Rap Genius

Meaning

Chance is talking about dipping his cigarettes in Promethazine with codeine, or lean; square is slang for a cigarette, most commonly heard in the midwest.

There was a point where I just did not care about my body. I just felt indestructible. I was smoking mad lean squares
Chance’s Complex interview

He is also using lean as a verb and square as a geometric shape; when one alters the angles of a square, you’re left with a rhombus

In the video below, Chance illustrates the peppering of a cigarette. http://youtu.be/DWZBchgVSvc?t=1m17s

Chance actually used a very similar line on an unreleased track called “Something Bout Us” off the album Good Enough from back in his Instrumentality days with J-Emcee.

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