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].
early 12c., "come into agreement," also "agree, be in harmony," from Old French acorder "agree, be in harmony" (12c.), from Vulgar Latin *accordare "make agree," literally "be of one heart, bring heart to heart," from Latin ad "to" (see ad-) + cor (genitive cordis) "heart" (used figuratively for "soul, mind"), from PIE root *kerd- "heart." Compare concord, discord. Related: Accorded; according.
early 15c., "performance of a task; state of completion," from Old French acomplissement "completion, action of accomplishing," from acomplir "to fulfill, carry out, complete" (see accomplish). Meaning "thing completed" and that of "something that completes" someone and fits him or her for cultivated or fashionable society are from c. 1600.
late 14c., "fulfill, perform, carry out an undertaking," from Old French acompliss-, present-participle stem of acomplir "to fulfill, fill up, complete" (12c., Modern French accomplir), from Vulgar Latin *accomplere, from Latin ad "to" (see ad-) + complere "to fill up," transferred to "fulfill, finish (a task)," from com-, here probably as an intensive prefix (see com-), + plere "to fill" (from PIE root *pele- (1) "to fill"). Related: Accomplished; accomplishing.
"associate in crime," 1580s, an unetymological extension of earlier complice "an associate or confederate" (early 15c.), from Old French complice "a confederate, partner" (not in a criminal sense), from Late Latin complicem (nominative complex) "partner, confederate," from Latin complicare "to involve," literally "fold together," from com "with, together" (see com-) + plicare "to fold, weave" (from PIE root *plek- "to plait"). Altered perhaps on model of accomplish, etc., or by assimilation of the indefinite article in a complice.