"Knuckled under" is a phrase commonly used to describe somebody who has surrendered or given in to pressure or demands. There are a number of synonyms for this phrase, including "caved in," "acquiesced," "yielded," "submitted," and "complied." Other related terms that convey a similar idea include "conceded," "deferred," "capitulated," "surrendered," and "gave way." Each of these terms implies a sense of succumbing to pressure or relinquishing control, often in the face of overwhelming odds or persistent persuasion. While the exact meaning may vary slightly between these different synonyms, they all convey a similar idea of defeat or submission.