There are a number of different defintions for this phrase:
1) Possibly the best known, someone, male or female who indulges in cunnilingus also know as
muff diving.
2) Someone, usually a small child but can be an adult, who is prone to temper tantrums, throws themselves to the ground and bites the carpet.
3) Someone who is extremely eccentric, or even
clinically insane, who is prone to do things that range from slightly strange to outrageously weird.
4) A derogatory name for a person with learning difficulties, similar to
window licker.
1) Look at those two fat dykes! A right pair of rug
munchers!
2) There was this really nasty kid, screaming at his mother in the middle of the shop,
stamping his feet and trying to hit her! A thoroughly nasty little
rug muncher.
3) John's acting stranger and stranger. It's sad to see a gifted man turning into a rug muncher.
4) If I hear you calling a person with learning difficulties a 'rug muncher' again, there will be real trouble.