do something on the sly
on the sly
Secretly; in a clandestine or furtive manner. Even though he said he'd quit smoking, he still kept up the habit on the sly. The senator had been taking bribes from lobbyists on the sly for years before he was caught.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
do something on the ˈslydo something secretly: She didn’t seem to have much appetite for dinner. I wonder if she’s been eating chocolates on the sly? OPPOSITE: (out) in(to) the open
Farlex Partner Idioms Dictionary © Farlex 2017