、。
Geez, what do you think you're doing?
Literal
Geez~, what doing-[explanatory-んだ]-[emphatic-よ]?
Informal exasperated speech. もう as an interjection = 'ugh/geez/come on!', expressing mild annoyance, often elongated as もう~ for emphasis. やってん is a casual contraction of やっているの → やってんの → やってん ('what are you doing?'). んだ marks explanation/emphasis, and よ adds insistence. A rhetorical scolding question.