What is Alien Legs?
From the movie the arrival when the aliens legs go in backwards. It is an action to give someone Alien Legs means to kick their legs in.
Hey your better shut the hell up or i'll give you alien legs.
A mafia-style torture, where the victims legs are made to bend at the knee the opposite way they usually do, making the victim look like an alien.
Christ, what did they do to his knees? Can he even walk?
Oh, you know, he was hanging with the wrong crowd, someone gave him the old alien legs.