。、。
I'm going to go buy underwear. You still don't have any on, do you?
Literal
Underwear [obj-を] buy-to go-[explanation-んだよ]. You, still no-pants is-[right-だろうが].
買いに行く is the standard 'go to do X' purpose construction: [verb ます-stem]+に+行く. ノーパン is a Japanese-English compound of 'no' + パン (short for パンツ, 'pants/underwear'), meaning 'not wearing underwear.' だろうが is an emphatic tag 'right, isn't it?' where the が adds confrontational force. An aggressive, matter-of-fact exchange — likely between siblings or close friends.