Etymology
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semi-demi- 

word-forming element meaning "sixty-fourth part," 1660s, a compound of semi- and demi-, both from Latin but unrelated etymologically. The compound prefix came to be used "vaguely in a diminutive sense" [OED]. Also compare demisemiquaver., and, of course, hemidemisemiquaver (1846), which also sometimes appears as semidemisemiquaver (by 1825).

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