From 《红楼梦》 chapter 58: Xiren says 一个巴掌拍不响, 老的也太不公些,小的也太可恶些 — faulting both the elder and the younger in a dispute.
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Very colloquial, used constantly in everyday arguments and mediation. Can be annoying to hear if you actually are the wronged party — it's the Chinese 'both sides' cliche.