An Etymological Dictionary of the German Language/Annotated/Kühl
Kühl, adjective, ‘cool,’ from the equivalent Middle High German küel, küele, adjective, also a regularly non-mutated form kuol- in compounds such as kuolhûs, neuter, ‘cooling-house,’ and in the adverb kuole (compare schon, spät, fast); Old High German chuoli, adjective, ‘cool’ (*chuolo, adverb). It corresponds to Middle Low German kôl, Dutch koel, Anglo-Saxon côl, English cool. In the form of kôli- (originally kôlu-) the adjective is common to West Teutonic; the adjective kalt is the old participle form from the stem of kühl, from which in Scandinavian (kala) and Anglo-Saxon (calan) strong verbs are formed; the further cognates English chill, Anglo-Saxon čęle, čyle, ‘cold,’ are based on a Gothic *kali- (neuter singular *kals). Compare kalt.