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.
See also: on, sly

do something on the ˈsly

do 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
See also: on, sly, something
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