Green’s Dictionary of Slang

a solicitor who takes an informer’s fee for their prosecution of the case.

1781		I. Jackman Divorce (1790) 10: I have this moment [...] received a letter from old Qui Tam, the Attorney.
1782		G. Parker Humorous Sketches 189: A lawyer [speaks of] John Doe and Richard Rowe, terms, vacations, quitams, processes and executions.
1796		Sporting Mag. Jan. VII 215/2: A furious rumpus has taken place among the knights of the gaming-table, owing it is said to the information abilities of the ingenious Bob Qui Tam having been treated with contempt, and refused the customary fees [...] Bob pledges himself to give due and regular information to the magistrates.
1811		Lex. Balatronicum n.p.: Qui Tam. A quitam horse; one that will both carry and draw. Law wit.
1830		W.T. Moncrieff Heart of London II i: craig.: Quitam Nosey. qui.: Here (Advancing in counsellor’s wig).
1839		C.J. Lever Harry Lorrequer 146: But law talk in all its plenitude, followed; and for two hours I heard of nothing but writs, detainers [...] and alibis, with sundry hints for qui tam processes.
1848		Flash Dict. in Sinks of London Laid Open 120: Qui tam, a shark, a lawyer.
1864, 1867, 1870		Hotten Sl. Dict. 210: qui-tam a solicitor. It properly means ‘who so,’ and is the title given to an action in the nature of an information on a penal suit.
1873		Sl. Dict. 265: Qui-tam a solicitor. He who, i.e., ‘he who, as much for himself as for the King,’ seeks a conviction, the penalty for which goes half to the informer and half to the Crown. The term would, therefore, with greater propriety, be applied to a spy than to a solicitor.