、。
I mean, really, what are you even saying — going sleeveless in this cold?
Literal
Basically you, this cold [though-のに] sleeveless [with-で] what are-saying [emph-のよ].
だいたい here is the rhetorical 'why in the world / basically why' opener of a complaint. あんた is the rough/informal 'you,' usually feminine-dismissive or familial. 寒いのに 'even though it's cold' uses the ~のに concessive. ノースリーブ 'sleeveless' is the stock loan for 'a sleeveless top,' a clothing-category wasei-leaning usage. 何いってんのよ is casual for 何を言っているのよ 'what are you saying, honestly!' — the feminine scolding の+よ.