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a man who is his own lawyer has a fool for a client
A man who tries to defend himself, rather than hiring a trained lawyer, is a fool. A: "What do you mean, a lawyer? I'm going to represent myself!" B: "Well, just keep in mind that a man who is his own lawyer has a fool for a client."
One who acts knowledgeable about something one actually knows little about. Stop being a guardhouse lawyer and giving me advice on how to get a job when you've been unemployed for months too!
A shrewd attorney, adept at dealing with legal technicalities, as in It would take a Philadelphia lawyer to get him off. This expression dates from the late 1700s and, as lexicographer Richard H. Thornton observed: "Why members of the Philadelphia bar should be credited with superhuman sagacity has never been satisfactorily explained."
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.
An adept attorney. The most probable reason why the City of Brotherly Love became an adjective for astute and skillful lawyers was Andrew Hamilton, whose 1735 defense of printer John Peter Zenger was a milestone of freedom of the press in America. (Lawyer Andrew should not be confused with Secretary of the Treasury Alexander Hamilton.) Although the Zenger trial was held in New York City, Hamilton was from Philadelphia. Curiously, it took some fifty years for the phrase to appear in print.