Basically, this is a saying used to describe something that a hell of a lot of people have.
For example, saying "everyone and their dog has a Ford Mustang" would be appropriate.
Or take the following conversation:
emo kid: "check out my ears, I just got them tapered, I'm so original"
person with an identity: "everyone and their dog has gauged ears in 2005"
73👍 10👎