Knee biter

Posted by Walter Lakey on December 16, 2003

ran across this phrase "knee biter" in one of the late Douglas Adams' Hitchhiker "trilogy". In context it was intended as an insult. It seems an odd thing to do or to be insulted by. Is this a euphemism for some less socially gracious insult? Or, is it just of Adams' silliness?
Thanks,
Walter