KTHX BYE: shortening 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 alteration of God be with you. "kthxbye" is the pinnacle of English's advancement, shortening "All correct, Thank you, God be with you." into seven lowercase letters. Humanity is doomed. Obviously, it is used to end a conversation *fast* that you don't want to be in anymore.