Brand new?

Or brand spanking new? You get so used to a phrase that you don't question it, but then today I did. My first search engine question was to ask is it only us Brits that say it?
But it certainly seems to have currency beyond the UK otherwise why would this South African marketing company have decided that it's brand would benefit from being a little bit spanky?
But what's it all about? I heard someone say the phrase today and when I gave it some thought the best explanation I could come up with was that it had something to do with the old tradition of the midwife slapping a newborn baby to get it to breathe.
But a bit of Googling suggests that that's way off the mark.This definition makes a connection between the s-word, strutting Scandinavians and horses.
While here there's a link made between the sort of branding that cowboys do to cows. Off topic, personally, I don't really understand the whole fiction niche that sees big, rough cowboy heroes spanking the living daylights out of bratty girls in gingham; maybe you have to be American to get it.

But anyway, so far 'brand new' makes some sort of sense. But why the spanking? Apparently it's about our favourite word having two meanings. 
Yes, there's the noun and verb one that involves the reddening of naughty bottoms, but there's more. Meaning number two is adj. 'exceptionally large or fine'.
Interesting. It turns out then that we've forgotten what it means, but we still like to use it. Personally, I think that just may be about the fact that all adult human beings have at least a little bit of spanko hidden deep inside and just like to have an excuse to say the word. 
And then there's all the people like you and me who don't hide TTWD too deeply and just love any opportunity to say 'spanking' in vanilla situations. Anyway, from now on I intend to make full use of both of my favourite word's two definitions.
I'm planning to let SO know that from today I want my spankings to be spanking - that is 'exceptionally large or fine'. Or preferably, exceptionally large and fine...