macking on the sly 

Covert flirting between two people. Because they have to keep their true feelings for each other secret, for whatever reason (i.e. they work together, one is married, etc.).

"Those two can't seem to stand each other! They're always fighting over something!"

"Wrong, man. It's all a front they put up. They're actually macking on the sly."

macking on the sly by D. Gould March 8, 2006