brawn (n.)

late 13c., "boar's flesh;" early 14c., "flesh of a muscular part of the body," from Old French braon "fleshy or muscular part, buttock," from Frankish *brado "ham, roast" or some other Germanic source, from Proto-Germanic *bred-on- (source also of Old High German brato "tender meat," German Braten "roast," Old Norse brað "raw meat," Old English bræd "flesh"), from PIE *bhre- "burn, heat," from root *bhreu- "to boil, bubble, effervesce, burn."

The etymological sense is "piece of meat suitable for roasting." "The specific sense 'boar's flesh' is exclusively of English development, and characteristic of English habits" [OED]. Meaning "well-developed muscles, muscular strength" is from 1865.