1
general::
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. [Mid1700s]
American Heritage Idioms
2
general::
Phrase(s): knuckle under (to someone or something)
to submit to someone or something; to yield or give in to someone or something. • You have to knuckle under to your boss if you expect to keep your job. • I’m too stubborn to knuckle under.
McGrawhill's American Idioms And Phrasal Verbs