Merriam-Webster's Collegiate® Dictionary, 11th Edition
RAT
I. noun see: rodent Date: before 12th century
1. any of numerous rodents ( Rattus and related genera) differing from the related mice especially by considerably larger size, any of various similar rodents, a contemptible person: as, one who betrays or deserts friends or associates, scab 3b, informer 2, a pad over which a woman's hair is arranged, a person who spends much time in a specified place , ratlike adjective
II. verb (ratted; ratting)
Date: 1812
intransitive verb to betray, desert, or inform on one's associates, to catch or hunt rats, to work as a scab, transitive verb to give (hair) the effect of greater quantity (as by use of a rat), to inform on ; turn in