What does "keen" mean?

Keen: Acute of mind; sharp; penetrating; having or expressing mental acuteness; as, a man of keen understanding; a keen look; keen features. To make our wits more keen. Shak. Before the keen inquiry of her thought. Cowper.

Additional senses

  1. 2.Bitter; piercing; acrimonious; cutting; stinging; severe; as, keen satire or sarcasm. Good father cardinal, cry thou amen To my keen curses. Shak.
  2. 3.Piercing; penetrating; cutting; sharp; -- applied to cold, wind, etc, ; as, a keen wind; the cold is very keen. Breasts the keen air, and carols as he goes. Goldsmith.
  3. 4.Eager; vehement; fierce; as, a keen appetite. "Of full kene will." Piers Plowman. So keen and greedy to confound a man. Shak. Note: Keen is often used in the composition of words, most of which are of obvious signification; as, keen-edged, keen-eyed, keen- sighted, keen-witted, etc.
  4. 5.To sharpen; to make cold. [R.] Cold winter keens the brightening flood. Thomson.
  5. 6.A prolonged wail for a deceased person. Cf. Coranach. [Ireland] Froude.
  6. 7.To wail as a keener does. [Ireland]

Sources