O.E. col, from P.Gmc. *koluz, from PIE base *gel- "cold, to freeze." The v. form kele (from O.E. colian) was used by Shakespeare, but has been assimilated with the adj. into the modern word. 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, said to have been popularized in jazz circles by tenor saxophonist Lester Young. Related: Cooling (mid-14c.); coolly (1570s).
1610s, "a vessel in which something is set to cool," agent noun from cool (v.). Meaning "insulated box to keep things cool" is from 1958. Slang meaning "jail" is attested from 1884.