Welcome aBoard, angelnanc, especially with such an iteresting question.

Over here in the UK, we say "slap-bang" in the middle - meaning precisely the same as the definition that you give.

I have no idea of its derivation, but personally associate it with the air raids of the Second World War, when bombs were said to drop, "slap-bang" in the middle of a factory, or housing estate or whatever. If this is where it started, the derivation is fairly clear, I think.