Found predominately in rednecks – when the lower teeth grow or extend past the upper teeth. This condition, if not fixed at a young age can cause digestion problems and also increase the risk of biting one’s own tongue.
Under manners is used in a jovial way when someone out with a group of mates ups & leaves because they told you to. Equivalent, but a little more insulting, than under the thumb. I believe it has a west indian/jamaican origin.