「はきゃあがって」＝「はきやがって」
The former is a very slangy, vulgar, tough guy's version of the latter. (Even the latter is already fairly vulgar.)
「はきやがって」＝「吐{は}く + やがる (in て-form)」
「セリフをはく」= "to utter a line"
「やがる」 is an auxiliary verb of contempt and hatred.
「かっこいいせりふをはきゃあがって・・」
thus means:
"You just utterd a cool line!" or
"That's a cool line you're speaking! "
"Hey, you're talkin' cool, mother****!" would be the actual nuance of it, TBH.
