on the sly
secretly, especially when you are doing something that you should not do

[TahlilGaran] Dictionary of Contemporary English ▲

on the sly
secretly.
She's been going out with him on the sly.

[TahlilGaran] English Idioms Dictionary ▲

on the sly
secretly
We went to the restaurant on the sly so that nobody would know where we were.

[TahlilGaran] English Idioms Dictionary ▲

on the sly
adv. phr. So that other people won't know; secretly.
The boys smoked on the sly.
Mary's mother did not approve of lipstick, but Mary used it on the sly.

[TahlilGaran] English Idioms Dictionary ▲