c. 1500, a back-formation from adoption or else from Old French adopter (14c.) or directly from Latin adoptare "chose for oneself, take by choice, select, adopt," especially "to take into a family, adopt as a child," from ad "to" (see ad-) + optare "choose, wish, desire" (see option (n.)). Originally in English also of friends, fathers, citizens, etc. Sense of "to legally take as one's own child" and that of "to embrace, espouse" a practice, method, etc. are from c. 1600. Related: Adopted; adopting.
word-forming element in legal English (and in imitation of it), representing the Anglo-French -é ending of past participles used as nouns (compare -y (3)). As these sometimes were coupled with agent nouns in -or, the two suffixes came to be used as a pair to denote the initiator and the recipient of an action.
Not to be confused with the French -ée that is a feminine noun ending (as in fiancée), which is from Latin -ata.