Most of the original senses of this word (“wise, brave, mighty, fierce”) had disappeared from English by Shakespeare’s time. But the notion of sharpness was common, used especially with reference to weapons, and also metaphorically to talk about winds, thoughts, words, and senses, where it expressed such notions as “biting, piercing, penetrating”. What has especially to be avoided is the modern sense of “eager” in the sense of “sexually attracted”. This is not what Ophelia means when she tells Hamlet “You are keen” (Hamlet, III.ii.257) or when Helena refers to an angry Hermia as “keen and shrewd” (A Midsummer Night’s Dream, III.ii.323). Here the ladies are using the word in its older meaning: “sharp, cutting, severe.” A milder sense is heard when Escalus says to Angelo “Let us be keen and rather cut a littleThan fall, and bruise to death” (Measure for Measure, II.i.5), where the primary nuance is “perceptive, shrewd”.