If you want to tell someone 'stop being stupid', that's ふざけるな, fuzakeru na. 'Drop dead' is くたばれ, kutabare. やかましい, yakamashii is basically 'shut up, you're noisy'--that's one of the first things I ever overheard Jotaro saying, so you know it'll be a coarse and unrefined way of talking.
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If you want to tell someone 'stop being stupid', that's ふざけるな, fuzakeru na. 'Drop dead' is くたばれ, kutabare. やかましい, yakamashii is basically 'shut up, you're noisy'--that's one of the first things I ever overheard Jotaro saying, so you know it'll be a coarse and unrefined way of talking.