Qui tam (generally pronounced key tam, rhymes with ham) is a shorthand version of the Latin phrase:
Qui tam pro domino rege quam pro se ipso,
which means
Who sues on behalf of the King, as well as for Himself.
It is the technical legal term that refers to the type of lawsuits brought under the False Claims Act, by a private citizen plaintiff on behalf of the federal government (rather than by the government itself).