knuckle under

British  

verb

1. (intr, adverb) to give way under pressure or authority; yield

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knuckle under Idioms  

1. Also, knuckle down. Give in, acknowledge defeat, as in The dean refused to knuckle under to the graduate students' demands, or He was forced to knuckle down before their threats of violence. Presumably this idiom alludes to a kneeling position with hands on the ground, knuckles down. [Mid-1700s]

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