coat of arms (n.) Look up coat of arms at Dictionary.com
mid-14c., originally a tunic embroidered with heraldic arms (worn over armor, etc); see from coat (n.) + arm (n.2) and compare Old French cote a armer. Sense transferred to the heraldic arms themselves by 1560s. Hence turncoat, one who put his coat on inside-out to hide the badge of his loyalty.