coolyoudaoicibaDictYouDict[cool 词源字典]
cool: [OE] Cool comes from the same source as cold, namely Indo-European *gel-, *gol- (from which English also gets congeal, gel, and jelly). The Germanic descendants of this Indo- European base were *kal-, *kōl-. From these were derived the Germanic adjective *kōluz, which passed into Old English as cōl. Its use for ‘fashionable, hip’ is mid-20th-century, but its nonchalant application to large sums of money is of surprisingly long standing: ‘I just made a couple of bets with him, took up a cool hundred, and so went to the King’s Arms’, John Vanbrugh and Colly Cibber, The Provok’d Husband 1728.
=> cold, congeal, gel, jelly[cool etymology, cool origin, 英语词源]
cool (adj.)youdaoicibaDictYouDict
Old English col "not warm" (but usually not as severe as cold), also, of persons, "unperturbed, undemonstrative," from Proto-Germanic *koluz (cognates: Middle Dutch coel, Dutch koel, Old High German kuoli, German kühl "cool," Old Norse kala "be cold"), from PIE root *gel- "cold, to freeze" (see cold (adj.)).

Applied since 1728 to large sums of money to give emphasis to amount. Meaning "calmly audacious" is from 1825. Slang use for "fashionable" is 1933, originally Black English; modern use as a general term of approval is from late 1940s, probably from bop talk and originally in reference to a style of jazz; said to have been popularized in jazz circles by tenor saxophonist Lester Young. Related: Coolly.
cool (n.)youdaoicibaDictYouDict
c. 1400, "coldness, coolness," from cool (adj.). Meaning "one's self-control, composure" (the thing you either keep or lose) is from 1966.
cool (v.)youdaoicibaDictYouDict
Old English colian, "to lose warmth," also figuratively, "to lose ardor," from the root of cool (adj.). Meaning "to cause to lose warmth" is from late 14c. Related: Cooled; cooling.