let up vs thwart

let up

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

  • To cease or stop. 

thwart

verb
  • To cause to fail; to frustrate, to prevent. 

adj
  • Placed or situated across something else; cross, oblique, transverse. 

noun
  • A seat across a boat on which a rower may sit. 

  • An act of thwarting; something which thwarts; a hindrance, an obstacle. 

  • A brace, perpendicular to the keel, that helps maintain the beam (“breadth”) of a marine vessel against external water pressure and that may serve to support the rail. 

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