14 May 2020
The phrase on the q.t. means in secret, confidentially. The q.t. is an abbreviation for quiet.
The phrase dates to the late nineteenth century. It’s first clearly attested to in 1885. It appears that year in George Moore’s novel A Mummer’s Wife:
Oh, I'm so glad; for perhaps this time it will be possible to have one spree on the strict q.t.
It also appears in the Sydney Bulletin on 18 July 1885:
Oh, my! what a pious world it is,
And how very good they all seem to be –
But what a ’duffing’ lot you’d find
If you would only raise the blind,
And see ’em on the strict Q.T.
The fact that it appears in both a British and Australian source in the same year, indicates that the phrase was already widespread by the time it first appears in print.
Farmer and Henley’s slang dictionary includes a citation that is supposedly from c. 1870 in the broadside ballad “The Talkative Man from Poplar,” by James McEvoy:
Whatever I tell you is on the Q.T.
But as far as I can tell, that song was not written until 1885, and the copy online at the British Library doesn’t contain the phrase. So, this citation appears to be inaccurate.
Sources:
Farmer, J.S. and W.E. Henley. Slang and Its Analogues, vol. 5, 1902, s.v. Q.T.
Green’s Dictionary of Slang, 2020, s.v. q.t. n.
McEvoy, James, “The Talkative Man from Poplar,” 1886. London, British Library MS H.1260.g.(41.).
Oxford English Dictionary, third edition, December 2007, s.v. q.t., n.3 and adj.