Search: on the qt
Why: Ross won't tell Monica & Chandler what he is planning for their wedding because he's keeping it "on the QT." As far as I know, QT only stands for "quality time," though the abbreviation term "on the DL" (down low) would be applicable here.
Answer: "On the quiet." There's no definitive source for the phrase, though it appears to be of 19th century British origin - not American. The longer phrase 'on the quiet' is also not especially old, but is first recorded somewhat before on the qt, in Otago: Goldfields & Resources, 1862:
Source: Phrases.org.uk"Unless men can work [the gold] on 'the quiet', they are not likely to make 'piles' so rapidly as Messrs. Hartley and Riley."
The More You Know: On my drive home from work tonight, I cracked myself up thinking about a guy trying to make money for dental school by moonlighting as a rapper named Toof D.K.
