Actually, its not a slang word, its an abreviation. Sometimes spelled "kthxbye", "kthnxbye", "kthxbi", "kthnxbi", etc. you get the point.
Its not a word, thats like saying pwn is a word, no its an ingorant way to spell own. Which is an ignorant way to say "Im better then you". I think someone accidently hit the p instead of the o((they ARE right next to eachother)), then everybody started doing that.