Similar to "too big for their boots" and implying that the person in question constantly talks about doing things which they would never have the courage to do.
Alex: Man I hate that prick Jared, next time I see him i'll beat his dick in the dirt.
Jonny: I doubt that dude, he hits the gym a whole lot and all you do is sit around playing Warcraft all day!
Jordan: Forget it Jonny, Alex just has a mouth too big for his balls.
Jonny: I doubt that dude, he hits the gym a whole lot and all you do is sit around playing Warcraft all day!
Jordan: Forget it Jonny, Alex just has a mouth too big for his balls.
by Jonny, Jared and Jordan October 19, 2010
Get the mouth too big for his ballsmug.