To conduct a strip club breach of etiquette such as taking pictures or touching dancers resulting in being asked to leave the establishment or being tossed.

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

so like when I'm imagining "sly" I just see a real slick looking dude just posted up against a suspicious building under a shadow.. zoom in and the fucker has a tooth pick in his mouth just so effortlessly sweet