Qui tam is short for a Latin phrase, Qui tam pro domino rege quam pro se ipso in hac parte sequitur, which translates as He who sues on behalf of the King, as well as for himself. Qui tam started in England, where in certain instances, the people were empowered to sue on behalf of the crown and receive a bounty for the recovery.
Of course, qui tam may be a short form, but for only six letters it gets pronounced all sorts of ways. For you trivia buffs, the correct pronunciation is the Latin one – kwee tam – rather than the kee-tam we tend to use if we are accustomed to Spanish or French. As for that a, its the same sound as in father, — so now you know!