yard-arm (n.)youdaoicibaDictgodict[yard-arm 词源字典]
also yardarm, 1550s, from yard (n.2) in the nautical sense (attested from Old English) + arm (n.1). In 19c. British naval custom, it was permissible to begin drinking when the sun was over the yard-arm.[yard-arm etymology, yard-arm origin, 英语词源]