knuckle under vs relinquish

knuckle under

verb
  • To yield or cooperate when pressured or forced to do so. 

relinquish

verb
  • To metaphorically surrender, yield control or possession. 

  • To give up, abandon or retire from something. To trade away. 

  • To let go (free, away), physically release. 

  • To accept to give up, withdraw etc. 

How often have the words knuckle under and relinquish occurred in a corpus of books? (source: Google Ngram Viewer )