could

Etymology

From Middle English coude, couthe, cuthe, from Old English cūþe, past indicative and past subjunctive form of cunnan (“to be able”) (compare related cūþ, whence English couth). The 'l' was added in the early 16th century by analogy with should and would; this was probably helped by the tendency for 'l' to be lost in those words (and so not written, leading to shudd, wode, etc).

verb

1. simple past of can
   Before I was blind, I could see very well.
2. conditional of can
   1. Used as a past subjunctive (contrary to fact).
      I think he could do it if he really wanted to.
      I wish I could fly!
   2. Used to politely ask for permission to do something.
      Could I borrow your coat?
   3. Used to politely ask for someone else to do something.
      Could you proofread this email?
   4. Used to show the possibility that something might happen.
      Even without hovering drones, a lurking assassin, a thumping score and a denouement, the real-life story of Edward Snowden, a rogue spy on the run, could be straight out of the cinema. But, as with Hollywood, the subplots and exotic locations may distract from the real message: America’s discomfort and its foes’ glee. 2013-06-29, “Travels and travails”, in The Economist, volume 407, number 8842, page 55
      We could rearrange the time if you like.
   5. Used to suggest something.
      You could try adding more salt to the soup.
3. (obsolete except Tyneside) past participle of can
   I haven't could sleep. 1981, Anthony Warner, English Auxiliaries: Structure and History, published 1993, page 222

noun

1. Something that could happen, or could be the case, under different circumstances; a potentiality.
   When the golf ball is there, the whole self-interference package — the hopes, worries, and fears; the thoughts on how-to and how-not-to; the woulds, the coulds, and the shoulds — is there too. 1996, Fred Shoemaker, Extraordinary Golf: The Art of the Possible, page 88
   Shushona you must learn to rightfully prioritize all the woulds, shoulds and coulds of your life. 2010, Shushona Novos, The Personal Universal: A Guidebook for Spiritual Evolution, page 395