1550s, "received," past-participle adjective from admit (v.). As "received as true or valid" from 1780 (implied in admittedly).
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admit v.
late 14c., admitten, "let in," from Latin admittere "admit, give entrance, allow to enter; grant an audience," of acts, "let be done, allow, permit," from ad "to" (see ad-) + mittere "let go, send" (see mission).
The sense of "concede in argument as valid or true" is recorded by early 15c. In Middle English the word sometimes also was amit, after Old French amettre, which was refashioned 15c. Related: Admitted; Admitting.