slacken vs unscrew

slacken

verb
  • To make slack, less taut, or less intense. 

  • To gradually decrease in intensity or tautness; to become slack. 

  • To deprive of cohesion by combining chemically with water; to slake. 

unscrew

verb
  • To loosen a screw or thing by turning it. 

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