auxiliary verb, godan verb (-ru)
have the nerve to; dare to
Attaches to the masu-stem of a verb to express strong contempt, hatred, or disdain for someone else's action. Highly derogatory and rough; used mainly in masculine, vulgar, or confrontational speech.
あいつ、よくもそんなこと言いやがったな。
That bastard had the nerve to say something like that.
何しやがるんだ、
What the hell do you think you're doing, you bastard!