Ago

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Middle English ago, agone, past participle of the obsolete verb ago ‘pass’, used to express passage of time.

wiktionary

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From Middle English ago, agon(“passed”), past participle of agon(“to depart, escape, pass”), from Old English āgān(“to go away, pass away, go forth, come to pass”), from Proto-Germanic *uz-(“out”), *gāną(“to go”), equivalent to a- +‎ gone. Cognate with German ergehen(“to come to pass, fare, go forth”). Compare also Old Saxon āgangan(“to go or pass by”), Gothic 𐌿𐍃𐌲𐌰𐌲𐌲𐌰𐌽( usgaggan, “to go forth”).

etymonline

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ago (adj.)

"gone, gone by; gone away," early 14c., a shortened form of agon "departed, passed away," past participle of a now-obsolete verb ago, agon "to go, proceed, go forth, pass away, come to an end," from Old English agan. This was formed from a- (1) "away" (perhaps here used as an intensive prefix) + gan "to go" (see go (v.)).

As an adverb, "in past times" (as in long ago) from late 14c. The form agone is now obsolete except as a dialectal variant.