jack-a-leg
A person you don't know and you hate. You call them a jack-a-leg when you're trying to talk about them but can't remember their name.
That jack-a-leg in the back of the class keeps talking to his friend and won't shut up.
My coworker is a jack-a-leg, he never does his job and always messes things up.
That jack-a-leg at the store took my money and gave me the wrong bag.