rappie, n. — Green’s Dictionary of Slang

rappie n.

[both men have faced or would face the same charge or rap n.¹ (3d); they would also talk or rap v.¹ (1e) together when plotting]

(US prison) a confederate.

1981		Maledicta V:1+2 (Summer + Winter) 265: A convict seeking a rappie wants to enlist a partner in committing a crime.
1992		Bentley & Corbett Prison Sl. 41: Rappie When two or more people are involved in the same crime, convicted and sentenced to prison for that crime.