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1.
any person that fakes in your face and go behind your back tell all kinds of lies and make up rumors about a person.
cIARA TOWNES IS A RAT MOUTH SHE GOES AROUND TOWN CLAIM COOL WITH DIFFERENT PEOPLE AND THEN SITTING UP GOSSIPING RUMORS ABOUT THEM,CLAIMING SHE THE REAL DEAL.
by KALASTARR September 23, 2009
 
2.
The feeling left in the mouth and throat after a heavy nights drinking. Impossible to brush out, eat out, sleep out or get rid of in any way.
Rob: Alright Olli, what you get up to last night.

Olli: Got fucking rat assed on vodka at some party then like passed out on some girls bed.

Rob: Sounds heavy

Olli: Yeah man I have serious rat mouth right about now
by DeeTown November 07, 2008
 
3.
The Rat Mouth is a mostly noctournal mammal who will mate with anything with a pulse. It's natural habitat encompasses all local watering holes and consumes copious amounts of alcohol. They are often forced to habitually migrate from one hole to another due to Alligator Mouth (see definition below), slander and libel (grafitti) to others and other henious acts. The Rat Mouth will puff up and instigate then call in reinforcements and cowardly call the law when they can no longer handle difficult prey. It will have the law on speed dial and inform the activities of it's former watering hole. Extreme caution should be used if you encounter a Rat Mouth. Do not befriend it, it is parasitic in nature and when all resources are exhausted it will turn on you with the above traits.
Watch out for her. She has a huge Rat Mouth and can not be trusted.
by People Watchers March 20, 2014