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1. See COOL DOWN. 2. Also, cool down or out. Calm down, become less ardent, angry, or agitated, as in We can't discuss it until you've cooled off. The verb cool alone has been used in this sense since approximately A.D. 1000; off and down were added in the late 1800s, and Davy Crockett's Almanac (1836) had: "Resting a while, just long enough to cool out a little." 3. Also, cool out. Kill someone, as in They threatened to cool off his brother. [Slang; first half of 1800s] Also see COOL OUT, def. 2.
American Heritage Idioms
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Phrase(s): cool off [and] cool down
1. Lit. to lose or reduce heat. • I wish my soup would cool off. I’m hungry. • It’ll cool down this evening, after dusk. 2. Fig. to let one’s anger die away. (As the "heat" of anger declines.) • I’m sorry I got angry. I’ll cool off in a minute. • Cool off, Tom. There is no sense getting so excited. 3. Fig. to let one’s passion or love die away. (As the "heat" of passion declines.) • Ted: Is Bob still in love with Jane? Bill: No, he’s cooled off a lot.
McGrawhill's American Idioms And Phrasal Verbs