WordWeb Online

1. Become slow or slower
   "Production slowed up";
   - slow, slow down, slack, slacken
    
2. Cause to proceed more slowly
   - slow, slow down
    
3. Lose velocity; move more slowly
   "The car slowed up";
   - decelerate, slow, slow down, retard

Derived forms: slows up, slowed up, slowing up

Type of: decrease, diminish, fall, lessen, weaken

Antonym: quicken