1866, "to confine within set limits," from French interner "send to the interior, confine," from interne "inner, internal" (14c.), from Latin internus "within, internal" (see internal; also compare intern (n.)).
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.
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Harper, D. (n.d.). Etymology of internee. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/internee