Quoting urbandictionary.com (http://www.urbandictionary.com/define.php?term=kthxbye):


kthxbye: 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.