Dog (noun)

1. A domesticated carnivorous mammal that is widely kept as a pet or for work or field sports.
2. A person who is considered to be unpleasant, disloyal, or of poor quality.
3. A device for gripping or holding, especially one for holding a tool or machine part.
4. A device for tracking or pursuing, such as a hound or bloodhound.
5. A something that is difficult or impossible to shake off.

Origin:

Middle english, from old english docga, probably of scandinavian origin; akin to old norse doggr.

Examples:

1. The dog barked at the mailman.
2. He is a lazy dog.
3. The wrench has a dog for tightening bolts.
4. The bloodhound is a good dog for tracking scents.
5. The problem was a dog that wouldn't let go.