Hidden

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Old English hȳdan, of West Germanic origin.

wiktionary

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Morphologically hid +‎ -en.

etymonline

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hidden (adj.)

past-participle adjective from hide (v.1); a Middle English formation (Old English had gehydd "hidden") on the model of ride/ridden, etc. As "secret, occult" from 1540s. Hidden persuaders (1957) was Vance Packard's term for ad men.