q.t. is a jokey short form of quiet, and you almost always meet it in the phrase on the q.t., meaning secretly — you tell someone something, but you do not want it passed around. It has a slightly old, wink-wink feel, like something a parent or a character in an old film might say: 'Keep this on the q.t.' People also write QT, without the dots. You would not put it in a serious school essay. Though actually, outside that one phrase the letters are rare. If you see qt in a recipe, it is more likely quart, a measure of liquid, which is a completely different thing.

悄悄地、保密（幾乎只出現在 on the q.t. 這個片語）