1
general::
Become genial or pleasant, calm down, relax, as in The teacher mellowed out when they explained what had happened. This expression uses mellow in the sense of "ripening," with the connotation of softness and sweetness. [Slang; late 1900s]
American Heritage Idioms
2
general::
Phrase(s): mellow out
1. to become less angry. • When you mellow out, maybe we can talk. • Come on, man, stop yelling and mellow out! 2. to become generally more relaxed. • Gary was nearly forty before he started to mellow out a little and take life less seriously. • After his illness, he mellowed out and seemed more glad to be alive.
McGrawhill's American Idioms And Phrasal Verbs