'Hemi' is used to describe an internal combustion engine wherein the top of each piston (not Walter) are convex shaped to focus/distribute more of the available power to the drive shaft.

'Semi' has become *our word for any truck with a removable cargo box.

'Demi' is the lady assistant to that guy who couldn't "Handel the truth".

It seems as if some tranference is involved in each yet nothing near a "half" and they clearly have nothing to do with music, Faldage.