。
Why are you moping around like that?
Literal
What sluggishly walking-[are-んだ]?
ちんたら is a mimetic adverb for 'slowly, sluggishly, draggingly,' often with criticism attached. 歩いてんだ is the casual contraction of 歩いているんだ ('are you walking (in that way)?'). The sentence-initial なに functions as 'what (the heck are you doing)' — a complaint interrogative, not literally asking what. A short, sharp, masculine reprimand.