Green’s Dictionary of Slang

cool v.3

[the chilliness of the corpse]
(US)

1. to beat up.

[US]Phila. Inquirer 16 June n.p.: An essential part of the ‘toughie’s’ vocabulary is the verb ‘to cream.’ This verb has synonyms which make its meaning plain: To ‘knock cold,’ to ‘cool,’ to ‘beat up on’ and to ‘take.’ It may be used in connection with anything which the speaker dislikes and is often applied to certain examinations. A thing or a person which has been ‘creamed’ has been successfully treated in a violent manner.
[US]E. Thompson Garden of Sand (1981) 212: No coon could cool James J.
[US](con. 1940s) E. Thompson Tattoo (1977) 68: Glenn [...] spoke of cooling the old man on the bridge.

2. (also put the cool on) to kill, to murder, to assassinate.

[US]DN V 328: Newfoundland Dialect Terms [...] cool v.t. Kill.
[US]Flynn’s mag. 6 Sept. n.p.: Eight stick-up Johnnies out of ten aren’t so hot about coolin’ a cop [DU].
[US]T. Thursday ‘Once Upon a Crime’ in Crack Detective Sept. [Internet] I wouldn’t be surprised if the guy who cooled Jerome was right in Ami City now.
[US](con. 1945) F. Davis Spearhead 126: I sure cooled that son of a bitch.
[US]P. Thomas Down These Mean Streets (1970) 31: I was thinking how much heart he had. But I ran toward him like my life depended on it; I wanted to cool him.
[UK]N. Smith Gumshoe (1998) 124: No headlines [...] reading ‘Police seek comedian to help in Enquiries. Student dead.’ The cooled Azinge obviously wasn’t hot.
[US]L. Heinemann Paco’s Story (1987) 184: That’s how you put the cool on gooks.

3. to die.

[US]T. Capote Breakfast at Tiffany’s 89: Christ, I nearly cooled.
C. & R. Mortimer letter 5 Nov. in Dear Lupin (2014) [ebook] Mrs Hislop’s mother cooled last week; she was a compulsive gambler.