Quoting urbandictionaryUrban dictionary.com:
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.