Brisket
A tough, collagen-rich beef cut from the lower chest that becomes tender and juicy when cooked low and slow.
A whole packer brisket has two muscles: the leaner flat and the fattier point. At backyard temps around 250°F to 275°F (121°C to 135°C), that connective tissue slowly turns into gelatin. That is why a 12 to 16 hour cook is normal, not a flex. For the full process, see Brisket 101.