brush up on vs rub up

brush up on

rub up

verb
  • To polish or scrub; to cover (something with a substance) by rubbing. 

  • To rub (a body part): to massage, give a massage to. 

  • To assault (someone). 

  • To create (something) by rubbing. 

How often have the words brush up on and rub up occurred in a corpus of books? (source: Google Ngram Viewer )