Compiled with insights from @alanakysar.
  1. Ok.
    Sometimes it really just means you've received the message and all is well. This is rare, and usually occurs only in emotionless or logistical contexts (ie., "Is it raining?" "No" "K").
  2. I'm so angry at you that the only word I can summon is not even a word.
    It's just a letter.
  3. Are you out of your mind?
    Back pedal, fast. I may sound like I'm agreeing with you, but I actually vehemently disagree.
  4. I hate what you just said, but ok.
    So much hatred, so few syllables.
  5. I wouldn't do/think/say that, but ok.
    That is the worst idea ever, but you do you.
  6. I have no time for you.
    Not even a full words-worth.
  7. Let's bang
    Suggested by @dave
  8. I can't think of what to say but I don't want you to think I'm ignoring you.
    Suggested by @sally
  9. I'm going to say K because I know you know what I'm thinking and which variety I mean above so let's just keep this message short and sweet
    Suggested by @jansonebwoodlee
  10. I'm driving and trying to break the law as little as possible so all I can eeek out is K
    Suggested by @lindsinger