Good catch, tusseymountain. I found this explanation: "The word 'besague' was rescued by the late Lord Dillon from the compilers of antiquarian glossaries who had confused it with the double-headed axe (bis-aigue or bisacuta)." --The Antiquaries Journal 19: 427 (1939)
The OED lists this word with besague which it defines as "a double-edged axe." But isn't a besagew a round piece of armor worn to protect the armpit between the spaulders and the cuirass? It is sometimes also called a roundel. I stumbled on this trying to find how to pronounce besagew.