It is a situation which a boy who is carrying some goods after successfully escaped from chasing by policemen.
なあ...わらってたんじゃわからねえよ。
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Sign up to join this communityColloquial form vs. "dictionary" form:
「わらってた」=「わらっていた」
「ん」=「の」
「じゃ」=「では」
Put together, 「わらっていたのでは」 means "if you just kept laughing".
「では」, in this context, is like 「だと」 in meaning -- "if".
「なあ...わらってたんじゃわからねえよ。」
thus, means:
"Y'know, I don't get the picture if you just keep laughing."