WordWeb Online

1. Become quiet or calm, especially after a state of agitation
   "After the fight both men need to calm down.";
   - calm, cool off, chill out, simmer down, settle down, cool it, wind down, take a chill pill
    
2. Make calm or still
   "calm down the dragons of worry and fear";
   - calm, quiet, tranquilize [N. Amer], tranquillize, tranquillise [Brit], quieten [Brit], lull, still
    
3. Become quiet or less intensive
   "the fighting calmed down for a moment";
   - lull

Derived forms: calms down, calmed down, calming down

Type of: change state, comfort, console, hush, hush up, quieten [Brit], shut up, silence, solace, soothe, still, turn