1.) Verb - When you thrust a rusty knife into a girls unsuspecting poon until the ass and poon become one. You then insert thy crusty penis into poon until ones penis is then exiting out the ass. Hence the shape of a boomarang (aka poonarang)
2.) Noun - the poonarang

A female that ,no matter how much you try to get rid of her, you end up sleeping with her again and again.