Green’s Dictionary of Slang

(US prison) a prison inmate who has made themselves into a self-taught lawyer, either to pursue their own case, combat prison corruption or help their fellow inmates.

1950		Goldin et al. DAUL 240/1: Writ-bug. (P) A convict who makes a practice of petitioning for writs of habeas corpus and other legal writs without sufficient cause.
1971–2		C. Shafer ‘Catheads [...] and Cho-Cho Sticks’ in Abernethy Bounty of Texas (1990) 217: writ writer, n. – anyone having knowledge of the law.
2000		Other Side of the Wall: Prisoner’s Dict. July 🌐 Writ-Writer: A jailhouse lawyer, or someone who writes a lot of law suits or prison grievance appeals.