Disarm

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late Middle English: from Old French desarmer .

wiktionary

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From Middle English desarmen(“to divest of arms”), from Anglo-Norman desarmer.

etymonline

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disarm (v.)

late 14c., "deprive of power to injure or terrify, render harmless," a figurative sense, from Old French desarmer (11c.), from des- "reverse of" (see dis-) + armer "to arm" (see arm (v.)). The literal senses "deprive of weapons" (transitive), "put off one's armor or lay down one's weapons" (intransitive) are early 15c. Related: Disarmed; disarming; disarmingly.