Shortening of kthxbye
, which is in turn a shortened form of "k thx bye". The K is short for OK, which is short for "oll korrect", which is a facetious alteration of "All Correct". Thx
is short for "thanx" which is a facetious alteration of thanks which is short for "Thank You". "Bye" is short for goodbye, which is an alteration of "God Be With You". "ktb" is the pinnacle of English's advancement, shortening "All correct, Thank you, God be with you." into three letters. Humanity is doomed. Obviously, it is used to end a conversation *fast* that you don't want to be in anymore.
Whiny bitch: Here's your hamburger. Anyway, as I was saying about how bad my life is...
Other dude: Yum, burger. ktb. *leaves*
Whiny bitch: I have no friends. Wan wan wan.