Other forms: dogs; dogged; dogging
A dog is a member of the Canine genus, sometimes found wild but often found curled up on the couch when you're not at home.
A dog is an animal of the genus Canis, probably descended from wolves, but in some cases more like teddy bears. Animal names are really old words, and dog is old enough that no one knows where it's from. We also call ugly ladies and rotten gentlemen "dogs," so be careful not to call someone "a dog" when you mean, "what up, dog." It's also a verb: to dog (or to hound) someone is to follow them around relentlessly.