"knock yourself out" is never used in the English language in the way it's presented in that Urban Dictionary?
Without looking at Urban dictionary (valuable at times though there's some very weird stuff there)....
"knock yourself out" means 'go for it' (śmiało?) but also has a second part that isn't spoken, the second part of the meaning is "I think it's a dumb idea" and/or "leave me out of it"
My proposition for a Polish swear word
real word to be retooled for swearing: cążki (Uciekł mi autobus.... cążki!)
second choice: halama, name but it sounds vaguely ominous... (Mogę dwpaść? Lepiej nie... halama u mnie)
I thought of a made-up word.... lutra
Słyszałeś? Magda ma nowego chłopaka
To czwarty w tym roku... co za lutra!
But apparently it already exists....