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slang An inmate of a prison or jail who is or has become knowledgeable enough about the law to provide legal advice or representation to themselves or others. I couldn't afford to hire a lawyer, and the state was too slow providing me with one, so I had to rely on a couple of jailhouse attorneys I met in the slammer to help me file my appeal. I really would recommend hiring a real lawyer. You shouldn't be basing your case on what some jailhouse attorney is telling you.
Someone who has not formally studied law but knows enough about it to be able to help others with legal issues (as a prison inmate experienced in dealing with the law might). Despite the name, this phrase can be used in settings other than jail or prison. Talk to Sal before your court appearance—he's a real jailhouse lawyer.
A non-attorney who dispenses legal advice. Properly speaking, a jailhouse lawyer is a prison inmate who, although not a law school graduate (much less a member of the bar), has the requisite skill to assist other prisoners with such legal matters as preparing and filing appeals, writs, and pardon requests. Much of such knowledge came from personal experience. The phrase also applies to any layman, behind bars or not, who offers legal advice, solicited or not.