Oh, I see. Perhaps Susan can add to this, but in plain English, it is a "chap" or "fellow" or in a slightly different context, it is a kind of fellow who talks a lot about everything, rather like a
"Know---It---All". The person may be disliked because he talks, and talks, and talks. So his mouth movements might be similar to a dog wagging his tail constantly, Back--And---Forth.
Some of the personalities in the News Media can be referred to as a "wag". In the same sense, they are also referred to as "Talking Heads". It is just some "head" that is there going "Blah, blah, blah". We might use WAG in a sentence like this.
"Some wag was going on and on and on about the financial crisis until I just wished he would shut up!"
or, "Some wag appeared in a pop-up on my computer going on and on about the best investments and the impending econonmic collapse of the whole world. Good grief! What a nut! Wags like that just hack me off!" (make me angry)