let up in WordNet English dictionary

1. become less in amount or intensity
   "The storm abated"; "The rain let up after a few hours"
   abate  let up  slack off  slack  die away 
   
2. reduce pressure or intensity
   "he eased off the gas pedal and the car slowed down"
   ease up  ease off  let up 
   

let up in English Wiktionary