let up vs take a rain check

let up

verb
  • To cease or stop (doing something intense). 

  • To cease or stop. 

take a rain check

verb
  • In social interactions, to request a deferral of an invitation. To "take a rain check" is a polite way to turn down an engagement, usually with the implication that one is simply postponing it and another time might be acceptable. 

  • To request or accept a rain check (an agreement from a merchant to honor a special offer, temporarily unavailable, after the expiration date). 

How often have the words let up and take a rain check occurred in a corpus of books? (source: Google Ngram Viewer )