arm (v.) Look up arm at Dictionary.com
"to furnish with weapons," c. 1200, from Old French armer or directly from Latin armare, from arma (see arm (n.2)). Related: Armed; arming.
arm (n.1) Look up arm at Dictionary.com
"upper limb," Old English earm "arm," from Proto-Germanic *armaz (cognates: Old Saxon, Danish, Swedish, Middle Dutch, German arm, Old Norse armr, Old Frisian erm), from PIE root *ar- "fit, join" (cognates: Sanskrit irmah "arm," Armenian armukn "elbow," Old Prussian irmo "arm," Greek arthron "a joint," Latin armus "shoulder"). Arm of the sea was in Old English. Arm-twister "powerful persuader" is from 1938. Arm-wrestling is from 1899.
They wenten arme in arme yfere Into the gardyn [Chaucer]
arm (n.2) Look up arm at Dictionary.com
"weapon," c. 1300, armes (plural) "weapons of a warrior," from Old French armes (plural), "arms, war, warfare," mid-13c., from Latin arma "weapons" (including armor), literally "tools, implements (of war)," from PIE root *ar- "fit, join" (see arm (n.1)). The notion seems to be "that which is fitted together." Meaning "heraldic insignia" (in coat of arms, etc.) is early 14c.; originally they were borne on shields of fully armed knights or barons.