WordWeb Online

1. Become slow or slower
   - slow, slow down, slow up, slack
    
2. Make less active or fast
   "He slackened his pace as he got tired";
   - slack, slack up, relax
    
3. Become looser or slack
   "the rope slackened"
    
4. Make slack as by lessening tension or firmness
   - remit

Derived forms: slackening, slackened, slackens

See also: slacken off

Type of: decrease, lessen, loose, loosen, minify, weaken