any of numerous long-tailed murine rodents, esp of the genus Rattus, that are similar to but larger than mice and are now distributed all over the world
See also brown rat, black rat
2. informal
a person who deserts his or her friends or associates, esp in time of trouble
3. informal
a worker who works during a strike; blackleg; scab
4. slang, mainly US
an informer; stool pigeon
5. informal
a despicable person
6. smell a rat
verbWord forms: rats, ratting or ratted
7. (intransitive; usually foll byon) informal
a.
to divulge secret information (about); betray the trust (of)