Is 'ayu' English, or is it actually Japanese?

'Ayu' is a word borrowed from Japanese 'ayu'.

ayu

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note: A sweetfish - an amphidromous fish of East Asia - the only member of its genus and family - Plecoglossus altivelis. Named and prized for its sweet-tasting flesh. From Japanese (ayu: sweetfish).
Etymology: -

See more loan words from (unknown date).