Green’s Dictionary of Slang

Q.T. adj.

[on the Q.T. under Q.T. n.]

surreptitious.

[US]‘Iceberg Slim’ Airtight Willie and Me 123: Pad rent for her junkie brother and her Q.T. sweetie.
[UK]J. Morton Lowspeak.
[US]L. Pettiway Honey, Honey, Miss Thang 215: In Falk Corrections Center. And I had a husband know [sic] as Q.T. Gorgeous daddy.