When someone copies your hard work.
Usually used when referencing graphic design work that has been appropriated without permission.
The stealing of someone's work and trying to pass it off as your own. Followed by an excuse that gives the history of the stolen design but doesn't actually own up to it or even refer to it.
"Wow, he yasly'd you hard!"
"That t-shirt design was completely yasly'd, he stole the entire thing!"