Someone or somebody who has a machine gun mouth babbles idly and rapidly, to some extent, non-stop.

A person with a machine gun mouth is very annoying that you feel like wanting to punch his or her face to make him/her stfu.
YouTube video guy whom speaks very fast: Hello. Today I'll be showing you how to look more dapper........(10 minutes later)............so that you will be fancied by girls around you.

Me: Oh shit! I don't get what he is trying to say, this machine gun mouth guy!!
by Le Mao February 10, 2013
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