According

google

ref

Old English, from Old French acorder ‘reconcile, be of one mind’, from Latin ad- ‘to’ + cor, cord- ‘heart’; influenced by concord.

wiktionary

ref

accord +‎  -ing

etymonline

ref

according (adj./adv.)

c. 1300, "matching, similar, corresponding" (a sense now obsolete), present-participle adjective and adverb from accord (v.). Meanings "conforming (to), compliant, in agreement; consistent, harmonious; suitable, appropriate" are from late 14c. According to "referring to," literally "in a manner agreeing with" is from late 14c. As an adverb, "often applied to persons, but referring elliptically to their statements or opinions" [Century Dictionary].