Etymology
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armless (adj.)
late 14c., of physical conditions, from arm (n.1) + -less. Meaning "without weapons" is attested from 1610s (from arm (n.2)), but that sense more typically is expressed by unarmed or disarmed.
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forearm (n.)
between the elbow and the wrist, 1741, from fore- + arm (n.1).
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underarm (adj.)
1816, "underhand" (in reference to a style of throwing), from under + arm (n.1). First attested 1908 in dressmaking sense of "seams on the lower half of the arm-hole;" as a euphemism for armpit, it is attested from 1930s, popularized by advertisers.
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armed (adj.)
"equipped for battle," early 13c., past-participle adjective from arm (v.).
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dexter (adj.)

1560s, "pertaining to or situated on the right hand," from Latin dexter "on the right hand" (source also of French dextre, Spanish diestro, Italian destro),  from PIE root *deks- "right, opposite of left; south." The Latin form is with the comparative suffix -ter, thus meaning etymologically "the better direction." Middle English dester meant "right hand," and compare destrier. In heraldry, the part of the shield which is to the right when fitted on the arm, hence the side of the field to the left of the spectator.

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armament (n.)
1650s, "naval force equipped for war," from Latin armamentum "implement," from Latin armare "to arm, furnish with weapons" from arma "weapons" (including defensive armor), literally "tools, implements (of war);" see arm (n.2). Meaning "process of equipping for war" is from 1813.
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rearm (v.)

also re-arm, "provide with a new supply of weapons; acquire a new supply of weapons," 1805 (implied in rearming), from re- "back, again" + arm (v.) "to take up arms; supply with arms." In 20c., especially "to acquire more advanced weapons." Related: Rearmed.

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sidekick (n.)
also side-kick, "companion or close associate," 1901, also side-kicker (1903), American English, of unknown origin. Earlier terms were side-pal (1886), side-partner (1886).
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forearm (v.)
"prepare for an attack," 1590s, from fore- + arm (v.) "take up weapons." Related: Forearmed; forearming.
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armadillo (n.)
burrowing mammal of the American tropics, 1570s, from Spanish armadillo, diminutive of armado "armored," from Latin armatus, past participle of armare "to arm, furnish with weapons," from arma "weapons" (including defensive armor), literally "tools, implements (of war);" see arm (n.2). So called for its hard, plated shell.
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