It was said by a guy who has just realized that his kid has done something improperly.
Here is the sentence including the word.
The word here is やる 'do'.
連用形 + やがる is a way of demeaning the participator in an action. Much like -て + しまう, it indicates discontent with the fact that the action happened; but unlike with しまう, the discontent is directed primarily at the person who chose to do the action (rather than しまう's focus more on the typically unintended action itself). It can be rendered with phrases like 'had the gall to do', 'went and did', etc; though often it works to just leave it untranslated.