What does "cool off" mean?

see cool down.

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.”

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.