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Used in NZ/Australia to denote the middle of the week - from memory, James is from WA (Western Australia) for those of you from the Northern Hemisphere ...
There are some interesting other local terms (I'm think in particular of Tasmania) that no one other than locals would get.
I can't find exactly from where, but it originated in North America. Possibly derived from the idea of 'getting over the hump' as achieving a difficult task. As ever there are posts on the inter-web that claim definitive origins, but with no sources nor other evidence, putting it somewhere in the 50s or 60s (perhaps).
No doubt people from North America who look at this forum may have some more solid, etymological information.