Assistant

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late Middle English: from Old French, or from medieval Latin assistent- ‘taking one's stand beside’, from the verb assistere (see assist).

wiktionary

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From Middle French assistant, from assister.

etymonline

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assistant (n.)

mid-15c., assistent "one who helps or aids another," from Latin assistentem (nominative assistens), noun use of present participle of assistere "stand by, attend" (see assist (v.)). The spelling changed in French then (16c.) in English.

assistant (adj.)

mid-15c., "helpful, of assistance," from Latin assistentem (nominative assistens), present participle of assistere "stand by, attend" (see assist (v.)). The spelling changed in French then (16c.) in English.