WordNet (r) 3.0 (2006):

true bill
    n 1: an indictment endorsed by a grand jury

Bouvier's Law Dictionary, Revised 6th Ed (1856):

TRUE BILL, practice. These words are endorsed on a bill of indictment, when
a grand jury, after having heard the witnesses for the government, are of
opinion that there is sufficient cause to put the defendant on his trial.
Formerly, the endorsement was Billa vera, when legal proceedings were in
Latin; it is still the practice to write on the back of the bill Ignoramus,
when the jury do not find it to be a true bill. Vide Grand Jury.