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