slacken vs weaken

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. 

weaken

verb
  • To make weaker or less strong. 

  • To become weaker or less strong. 

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