WordNet (r) 3.0 (2006):

chill out
    v 1: become quiet or calm, especially after a state of
         agitation; "After the fight both men need to cool off.";
         "It took a while after the baby was born for things to
         settle down again." [syn: calm, calm down, cool off,
         chill out, simmer down, settle down, cool it]