To be acting in a sly or clever manner.

To do something on the sly is doing something sneakily, like a ninja.

Shorter, easier way of saying
'on the sly' meaning 'in secret' or 'not obviously'.

Usually pronounced like 'ont sly' by people from the north of england.

To do something cunningly, preferably without any one seeing, apart from your best mate to prove you did it

abbreviation for "No, seriously"

secretive, being very smooth or mischevious

Very sad, awful, wrong or not fair.