Green’s Dictionary of Slang

(US black/prison) to agree; to underwrite or verify.

1992		Bentley & Corbett Prison Sl. 14: Co-sign means to verify something. It can be qualifying a person’s credit or verifying something someone has said.
2001		(con. 1975–6) E. Little Steel Toes 10: He says he’s gonna cosign Monkey, we move on his guy e gotta move on all of them.