jailhouse lawyer

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A prison or jail inmate who is self-taught about the law and offers information to other inmates, especially to assist them in cases in which they are defending themselves.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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.