<grin>

In UK & Oz & NZ, the word dummy has many meanings including the ones Bill mentioned. Perhaps that is why we like playing with it so much.

Dummy is a pacifier
Dummy is a 'soft' idiot (very mild definition, almost a term of endearment as in - "don't be such a dummy")

Am sure it is derived from Dumb.

As to the expressions
"Go suck a dummy" - that is a putdown that implies the target is infantile, or doesn't understand what is going on around them, or needs pacifying.

"He spat the Dummy" is a common expression that means the person when confronted with something difficult or unplesant, threw a tantrum or 'flew off the handle', reacted strongly & loudly.

"A dummy spit" then, is another way of describing a big reaction to some kind of confronting situation.

The derivation is that when a baby is crying one gives it a 'dummy' to suck in order to try & quiet it down, but a baby intent of sqawking or crying or screaming, spits the dummy out then performs per its intent (in effect a tantrum).

Cheers

Doug Marker