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1.
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
by coderkeitaro October 13, 2007
 
2.
a little, petty jerk that metaphorically doesn't even come up to your waist in stature.
"man, did you hear what Arminda said Pete?" "Yeah, she just a knee biter"
by ttocs May 29, 2004
 
3.
A person who is excessively obnoxious. Apparently a chiefly British usage, synonymous with git. One step above an anklebiter.
"Dent, you're a jerk... A complete kneebiter."
by Aravin July 21, 2005