Rat (noun)

1. A large, omnivorous rodent with a long tail and keen sense of hearing, found throughout the world and commonly kept as a pet or used in laboratory experiments.
2. An unpleasant or mean person.
3. An informer or spy, especially one working for the police or government.

Origin:

Middle english: from old french rat, from latin ratus 'calculated, rational'. the sense development from 'person who calculates' to 'person who betrays or informs on others' (mid 16th century) to 'rodent' (late 17th century) is via the idea of 'something that gnaws or chews'. the use as a term of contempt for a mean or unpleasant person dates from the early 17th century.

Examples:

1. The pet store sold a variety of rats as pets.
2. The lab used rats for their behavioral experiments.
3. The politician was often referred to as a rat for his sneaky and untrustworthy behavior.
4. The rat told the authorities about his accomplice's criminal activities.
5. The children were afraid of the large rat they saw in the alley.