Cool  —adj. 1 of or at a fairly low temperature, fairly cold. 2 suggesting or achieving coolness. 3 calm, unexcited. 4 lacking enthusiasm. 5 unfriendly (a cool reception). 6 calmly audacious. 7 (prec. By a) colloq. At least (cost a cool thousand). 8 slang esp. Us marvellous. —n. 1 coolness. 2 cool air or place. 3 slang calmness, composure. —v. (often foll. By down, off) make or become cool.  cool it slang relax, calm down.  coolly adv. Coolness n. [old english]