gristle

noun

1. (countable, uncountable)Cartilage; now especially: cartilage present, as a tough substance, in meat.

   “When you're chewing on life's gristle Don't grumble, give a whistle And this'll help things turn out for the best[…]”

   “But she was tired of cell culture, tired of meticulously cutting away dead tissue like gristle from a steak […]”

2. (countable, figuratively, uncountable)Bone not yet hardened by age and hard work.

   “And it is a hard and cruel thing thus in early youth to taste beforehand the pangs which should be reserved for the stout time of manhood, when the gristle has become bone, and we stand up and fight out our lives, as a thing tried before and foreseen; for then we are veterans used to sieges and battles, and not green recruits, recoiling at the first shock of the encounter.”

   “1885, Ada Sarah Ballin, The Science of Dress in Theory and Practice, It. must be borne in mind that the bones of a young infant are little more than gristle, and are liable to bend, and so become deformed.”

   “"The young 'un will make his way," said Belcher, who had come across to us. "He's more a sparrer than a fighter just at present, but when his gristle sets he'll take on anything on the list.”