jailhouse lawyer

noun1

1. •
   (informal) A person incarcerated in a prison, jail, etc., who has acquired significant knowledge of the justice system and who is able to represent himself in some legal proceedings and to provide legal advice to other prisoners.

   • As something of a jailhouse lawyer, he has gathered affidavits attesting to abusive treatment by guards at the center, informed detainees of their legal rights and helped them write letters of appeal. — 1994