on the down-low

prepositional phrase3

1. •
   Of a man, publicly identifying as heterosexual but secretly having sex with men.
2. •
   (US, slang) In secret or nearly so, on a need-to-know basis.

   • I'll tell you, but keep it on the down-low.
3. •
   (US, slang) Secretly sleeping with someone other than one's partner.

   • And keep it on the down low Nobody has to know — 1995