an auxiliary verb attached to the masu-stem that expresses the speaker's contempt or anger toward someone's action; derogatory and colloquial.
あいつ、また遅刻しやがった。
That guy was late again (used with contempt).
そんなことをしやがって、許せない。
How dare they do something like that—unforgivable.
見やがったな。今に仕返ししてやる。
So you looked—I'll get back at you.