slacken in WordNet English dictionary

1. make less active or fast
   "He slackened his pace as he got tired"; "Don't relax your efforts now"
   slack  slacken  slack up  relax 
   
2. make slack as by lessening tension or firmness
   slacken  remit 
   
3. become slow or slower
   "Production slowed"
   slow  slow down  slow up  slack  slacken 
   
4. become looser or slack
   "the rope slackened"
   slacken