Derived from the term 'on the sly' - To be on the sly, the active of sly. You can do something on the sly, maybe stealing something funny and legendary, like mine and Jimbo's road bollard. You could also have 'slying it'
"Hey dude, I looked in the fridge just then and I cant find that steak that I had ready for dinner, Do you know where it went?"
"Not too sure bro, Sam was looking in there before. Maybe he took it on the ninja when no one was looking".