on the sly

In contravention of their oath, the men make advances on the sly and the wary women prank them to test their mettle.

We were forbidden to speak Urdu, but my mother still taught us on the sly.

He was paroled from detention but banned from work and national health insurance, though he worked on the sly until about eight years ago, when the government started to tighten controls over parolees.

During the height of Prohibition, people knew they could get booze on the sly: in speakeasies, say, or as “medicine” prescribed by pharmacists.

You steal work from US writers on the sly, and you're blacklisted.