sly one

A slang term used among close friends, usually males. Suggest sexual activities will take place of which nobody will know about.

Adam: I'm gonna go for a sly one in a bit ;)
Jamie: Eh? Who? Tell me!
Adam: I can't. It's a sly one.

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sly one off

Having a cheeky wank without anyone knowing.

Be right back I'm off to sly one off before my wife gets back