1. Talking about something or someone in a positive light, when in reality, the words coming out of your mouth are pure fiction.
2. Talking about something you don't know anything about while trying to present the illusion you are educated on the subject.
3. The act of saying absolutely ridiculous shit.
Joe: George Bush was the best president the United States ever had!
Jack: Imma punch you in your dumb face if you don't stop that mouthassjackery.
Vince Offer for the SlapChop:
... This thing, this tuna looks boring. Stop having a boring tuna, stop having a boring life. Add this tuna putting it in like this now you’re going to have a nice, tuna, salad. Look at this you’re gonna have an exciting life now.
TV Viewer: LOL - good ole' Vince ... He is the master of mouthassjackery.