A British broadside ballad (1870) contained the line “Whatever I tell you is on the Q.T.” This is the first record of Q.T. for “on the quiet, in confidence” recorded in English, but no one has established whether the broadside’s anonymous author was the first person to use the initals Q.T. to stand for quiet.

- from The Facts on File Encyclopedia of Word and Phrase Origins, by Robert Hendrickson