hold onto vs sit on

hold onto

verb
  • Used other than figuratively or idiomatically: see hold, onto. 

  • To manage to keep something. 

sit on

verb
  • Used other than figuratively or idiomatically: see sit, on. 

  • To take no action on; to hold in reserve without actually using. 

  • To hold an official inquiry regarding; to deliberate about. 

  • To block, suppress, restrain. 

  • To be a member of. 

  • To restrain (a person). 

How often have the words hold onto and sit on occurred in a corpus of books? (source: Google Ngram Viewer )