Keen1  adj. 1 enthusiastic, eager. 2 (foll. By on) enthusiastic about, fond of. 3 (of the senses) sharp. 4 intellectually acute. 5 (of a knife etc.) Sharp. 6 (of a sound, light, etc.) Penetrating, vivid. 7 (of a wind etc.) Piercingly cold. 8 (of a pain etc.) Acute. 9 (of a price) competitive.  keenly adv. Keenness n. [old english]