Apologies Shannon, I took "dumb them down" to mean the other meaning of dumb.

I have always heard that dummie is from dumb, but I may be mistaken as I cannot find anything on that.
Sorry, the literary side of me just had to check this one out!

I looked up my Oxford dictionary of word histories and the term 'dummy' actually was originally used in the sense of a person who could not speak and was 'an imaginary fourth player in whist' (18th century). Apparently this gave rise to 'a substitute for the real thing' (a rubber teat, a blank round of ammo) and then 'a model of a human being' (in the 19th century).

So in effect, when used to describe what we're talking about it's actual meaning is 'as a substitute for the real thing' (ie your nipple).

As a side point, did you know in Aboriginal culture it is still quite common for elderly women to calm babies by giving them their bb to suckle on? I guess if you can't pop down to the shop to buy a dummy you use the best thing available!