on the Q.T.

on the QT

Confidential(ly); secret(ly); without anyone else knowing. "QT" is an abbreviation of "quiet." Please keep this information on the QT until we are ready to make a public announcement. It turns out he's been writing a romance novel on the QT.
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Farlex Dictionary of Idioms.

on the Q.T.

Secretly, as in They told her on the Q.T. that she was being promoted. This slangy term, in which Q.T. is an abbreviation for "quiet," was first recorded in 1884.
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The American Heritage® Dictionary of Idioms by Christine Ammer.

on the q.t.

secretly or secret; without anyone noticing. informal
q.t. is a humorous abbreviation of quiet .
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Farlex Partner Idioms Dictionary

on the Q.T.

Confidentially. “Q.T.” is simply a shortened form of the word “quiet.” You're being told something on the Q.T. is to be asked to keep it under your hat.
See also: on
Endangered Phrases by Steven D. Price
See also:
  • bo
  • FYI
  • Y3
  • FWIW
  • shwoman
  • TMI
  • OTT
  • AIIC
  • WYM
  • phonetic