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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.
by Jonny, Jared and Jordan October 17, 2010
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