Green’s Dictionary of Slang

rappie n.

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

(US prison) a confederate.

[US]Maledicta V:1+2 (Summer + Winter) 265: A convict seeking a rappie wants to enlist a partner in committing a crime.
[US]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.