Meaning of the word "on the sly" in English

What does "on the sly" mean in English? Explore the meaning, pronunciation, and specific usage of this word with Lingoland.

on the sly

US /ɑn ðə slaɪ/
UK /ɒn ðə slaɪ/
"on the sly" picture

Idiom

doing something secretly or furtively

Example:
He was eating cookies on the sly when his mom wasn't looking.
They were meeting on the sly to plan the surprise party.
Related Word: