A secret engagement or solemn vow, typically between lovers, sealed by a ritual act of cutting one's arm as a pledge.
他们年轻时曾有过割臂之盟，私定终身，可惜后来因故未能走到一起。
When they were young, they made a "pledge of cutting arm" to marry each other secretly, but unfortunately, they couldn't be together later due to certain reasons.
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