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.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.