Green’s Dictionary of Slang

1. to leave the underworld, whether of pimping, drug sales and use or general criminality and devote oneself to a conventional lifestyle.

1968		N. Heard Howard Street 78: Soon’s I kick this habit I’m gon’ square up and git a job.
1977		J. Allen Assault with a Deadly Weapon 90: The halfway house did have some advantages if you really wanted to get involved and square up.
1990		S. Morgan Homeboy 55: We got what it takes to square up, Joe.

2. to persuade someone to take up a respectable lifestyle.

1969

‘Iceberg Slim’ Pimp 56: He had crossed her pimp into a five-year bit and squared her up.

3. to betray.

1972

D. Claerbaut Black Jargon in White America 81: square up v. to doublecross; betray.

4. to calm down.

1992

D. Burke Street Talk 2 50: You’d better square up or you’re gonna get into an accident drivin’ like that!