add vs let up

add

verb
  • To give by way of increased possession (to someone); to bestow (on). 

  • To append (e.g. a statement); to say further information; to add on. 

  • To summon minions or reinforcements. 

  • To combine elements of (something) into one quantity. 

  • To make an addition; to augment; to increase; to add on. 

  • To join or unite (e.g. one thing to another, or as several particulars) so as to increase the number, augment the quantity, or enlarge the magnitude, or so as to form into one aggregate. 

  • To sum up; to put together mentally; to add up. 

  • To perform the arithmetical operation of addition. 

noun
  • The addition of a song to a station's playlist. 

  • An act or instance of adding. 

  • An additional enemy that joins a fight after the primary target. 

let up

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

  • To cease or stop. 

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