Beware means to be on guard against somebody or something that may cause harm to you. It is usually put in a public places where the "dog" or other harmful objects are located, it warns people for them to be more attentive. I hope it serves good information to you.

Ha ha ha ha ha. It's a joke.

It is a joke. "Beware of dog" signs serve to warn people of a dog that has been know to be aggressive or bite. Some people use these signs, but there dog is not dangerous. they just use them to scare people and potential thieves. Because of this some signs make a joke of the "Beware of dog" sign. The most common is - "Beware of Cat" - as cat are usually not very harmful or dangerous. The man either actually hates his wife or has a good enough relationship with her to joke about her being a threat to him and others.

The writer of the board thinks that his wife is aggressive, at least towards extraneous people.