Green’s Dictionary of Slang

any amateur, esp. in the services or in prison, who considers themselves more expert in the law, esp. Queen’s Regulations or prison rules, than any professional and who will offer services, often to their detriment, to others.

1847		Private Soldiers 8: Do not gain for yourself the unenviable title of ‘lawyer’ — it is a very bad business in the army. [...] The loud, unmeaning talker is also much disliked and despised in a barrack room.
1864		Richmond Age (VA) Apr. 245/2: They [i.e. recruits from towns] are sharp, and well educated, but discontented, and too much of what are called ‘lawyers’.
1871		London Standard 31 Oct. 2/6: Jones threatened to take action against the witness and was very insolent [...] Jones was called by the nurses a barrack-room lawyer, for he was an old soldier.
1888		(con. 1866–71) H.H. McConnell Five Years a Cavalryman 196: Such a chap frequently becomes a nuisance to the officers, and is usually known in camp as a ‘guard-house lawyer.’.
1891		’Sailors’ Lingo’ in Hants. Teleg. 21 Feb. 11/3: To be cheeky is [...] to be a ‘sea lawyer’.
1895		C. King Ft Frayne 69: ‘Another guardhouse lawyer,’ said the first sergeant disgustedly.
1896		Lichfield Mercury 12 June 6/2: Major Savage had said that prisoner was ‘a bit of a barrack-room lawyer’ and advised him to be careful.
1907		J.A. Moss Officers’ Manual 243: guardhouse lawyer—a soldier with a smattering of knowledge of regulations and military law, quite loquacious and liberal with advice and council to men in the Guard House or other trouble.
1915		Reading Mercury 12 June 8/4: They thought he might be something of a ‘barrack-room lawyer’ and barrack-room lawyers [are] fought shy of.
1919		Hatchet (USS George Washington) 143: There is no place for the ‘guard house lawyer’ or the man who, on receiving an order, wonders whether the ‘old man’ knows what he’s talking about.
1921		Infantry Journal XIX 419/2: ‘But you can’t keep us at work after retreat,’ protested a guard-house lawyer.
1925		(con. WWI) Fraser & Gibbons Soldier and Sailor Words 140: Lawyer, A: A Service term for an argumentative or discontented man: anyone who has a grievance about his ‘rights’ and talks about them.
1926		Northampton Mercury 29 Oct. 2/6: There is a class of person who comes into Court, who is known in military circle as a ‘barrack-room lawyer’.
1935		H.S. Johnson Blue Eagle 39: I was by way of becoming a guard house lawyer — a very doubtful distinction in army circles.
1940		Phila. Eve. Ledger 20 July n.p.: ‘Guardhouse lawyer ’ – something like a sea lawyer, a man without authority who is always telling his fellows what their rights are and who usually is a trouble-maker.
1942		Yorks. Post 6 Mar. 5/4: Arthur Angus (17) [...] whom a military officer described asa ‘barrack-room lawyer’.
1953		Hartlepool Mail 16 July 8/2: I thought he was one of those truculent barack-room lawyer types.
1960		G.W. Target Teachers (1962) 201: ‘That man Miller is a menace,’ said Purney. [...] ‘What you would call a barrack-room lawyer,’ said Steve.
1970		J. Bouton Ball Four 375: The scout told him [...] that the word on me is that I’m a clubhouse lawyer.
1977		(con. 1941) R. Beilby Gunner 101: Sarn’t! This has gone far enough! I knew you were a barrack-room lawyer but I didn’t think you were a defeatist.