Dummy up

I have heard about a US slang, "dummy up", which actually means to keep quiet and thus, give no information, but I don't know how it originated. Can anyone answer this question?

Back before political correctness, dumb also meant mute, i.e. not able to speak. Thus dummy was a term for a mute person, mannequins, the puppets used by ventriloquists, etc. To dummy up is therefore to make yourself like an something or someone incapable of speech.