What you say when you don't want someone around. Namely the Cedric Diggory(ies, s?) of the world. Found in Harry Potter and the goblet of fire.
and more importantly, in the internet cult favorite musical, A Very Potter musical.
Harry/Darren: 'Cedric, seriosuly, screw off. You're just this spare guy who noone wants around. You're such a spare! "
Quirrel/Voldemort/Brian/Joe : KILL THE SPARE! AVADA KEDAVERA!