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1.
To make like a dummy - to SHUT UP!
"If the cops come around just dummy up."
by El Loudo February 10, 2005
 
2.
verb: to recommend to someone to keep quiet before they end up saying or doing something to prove their stupidity. The person often thinks they are undoudtedly correct. Often what they are saying refers to someone in their presence that they are unaware the story effects.
person 1: Hey, did you here that Joe got busted at Macy's ? (Joe's mom is right behind person 1 and overhears this story)

person 2 : Dude, why don't you dummy up !!

person 1 : Huh?
by ChrisL6464 April 10, 2010
 
3.
Smarten up
Quit being an idiot
Dummy-up and get out of the car.
by Death February 08, 2005
 
4.
Verb: to screw over, go back on your word, mess up, act stupid, say something retarded, piss some one off, be trippin on some one or basically do anythin that could make someone mad or think your stupid
Adjective: Someone who dummies up
Verb: That bitch just dummied the fuck up.
Adjective: You are soo dummy up right now.
That dude is DUMMY UP.
You dummy up asshole.
by lil nem May 25, 2006