The shape and markings on the blade as well as the overall shape of the entire knife is suggestive of a Zanzibar sword.
The flat, plain back is also in keeping with such pieces where only the front is patterned.
I'd be tempted to say a paper knife were it not for the blade, which makes me think that it may well be an elaborate dagger in the style of the Zanzibar swords and made to be carried in a frog.
No matter how you look at it, it's an interesting and unusual piece.
Mike
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