An Etymological Dictionary of the German Language/Kühl
Kühl, adj., ‘cool,’ from the equiv. MidHG. küel, küele, adj., also a regularly non-mutated form kuol- in compounds such as kuolhûs, n., ‘cooling-house,’ and in the adv. kuole (comp. schon, spät, fast); OHG. chuoli, adj., ‘cool’ (*chuolo, adv.). It corresponds to MidLG. kôl, Du. koel, AS. côl, E. cool. In the form of kôli- (orig. kôlu-) the adj. is common to West Teut.; the adj. kalt is the old partic. form from the stem of kühl, from which in Scand. (kala) and AS. (calan) str. vbs. are formed; the further cognates E. chill, AS. čęle, čyle, ‘cold,’ are based on a Goth. *kali- (n. sing. *kals). Comp. kalt.