A fictional insult constructed by the copy editors for the US edition of the Hitchikers Guide to The Galaxy series of books to replace the word asshole which was considered too offensive for the US readership.
A modern example of bowdlerization.
Original Sentence (UK edition of "Life, the Universe and Everything")
"You're a jerk, Dent... A complete asshole." - Wowbagger
Bowdlerized Version (US edition of "Life, the Universe and Everything")
"You're a jerk, Dent... A complete kneebiter." - Wowbagger
slang for the flesh on the back of the knee joint. this term usually references women who are so fat that the guy pursuing them cannot easily access her vagina and will instead fuck something easier to find, such as a belly fold, wrinkle, or in this case, the back of the knee.
you remember that big girl I was with last night? I couldnt get the pussy, but she had that BOMB kneeback!!