early 15c., assisten, "to help, aid, give assistance or support to in some undertaking or effort," from Old French assister "to stand by, help, put, place, assist" (14c.), from Latin assistere "stand by, take a stand near, attend," from assimilated form of ad "to" (see ad-) + sistere "stand still, take a stand; to set, place, cause to stand," from PIE *si-st-, reduplicated form of root *sta- "to stand, make or be firm." Related: Assisted; assisting. Medical assisted suicide attested from 1884.
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Harper, D. (n.d.). Etymology of sist. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/sist