In old English common law a citizen could bring a claim on behalf of the state and claim a reward if successful; the language refers to the introductory the Latin statement in such a writ,i.e., “qui tam pro domino rege quam pro se ipso in hac parte sequitur” or “so for the king as well as himself brings suit in this matter.”
The statute in the UK fell into disuse because it tended to be abused for sectarian purposes, but a US Federal qui tam law was adopted as the False Claims Act during the US Civil War to allow private citizens to police overcharging of the Federal Government for supplies and as amended remains in force (31 U.S.C. §§ 3729-3733.) It allows private parties to bring “whistleblowing” cases on behalf of the US government and receive a bounty of 15-25% of the recovery for the state. Qui Tam cases can be an issue in US government technology contracts.