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she'll/they'll/you'll etc hear from me spoken used for saying that you will be angry with someone, and you will tell them this 我会让她/他们/你（们）等好看的 He'd better apologize to you or he'll be hearing from me! 他最好向你道歉，否则我会要他好看的！
Synonyms and related words
Ways of threatening someone: watch it, or else, if someone isn't careful...