I know exactly what it means and I've heard it used to describe a jockey's toughness numerous times. I watch the Kentucky Derby with friends every year; and occasionally some other horse races. I don't care about The Derby, and I don't like horses much, but it's a fun time. Okay, jockeys are tough guys/girls. So are so many others. I'm simply saying that to argue that person X or person Y is the toughest, pound-for-pound, is 100% opinion-based and isn't something anyone needs to try and make a all-encompassing claim to. There you go, jockeys are very, very tough. They ride horses, which makes them tough. Stone masons are tough, too. They lift heavy rocks all day and they develop crazy callouses on their tough, strong hands. So are rock climbers. Many rock climbers are very lightweight, as it makes climbing rocks that much easier. If you are set on a pound-for-pound competition, start with them. Not people who get on the backs of horses and ride them. LOL, obviously I struck a nerve for you. Do you have a horse? I grew up with horses, but I don't like them much. One of them kicked my brother in the face for absolutely no reason. That horse was put down that day. I saw my dad shoot it in the head after my mom distracted it with some hay. It was traumatizing for me and I got smacked for sneaking out and watching it. That is not why I don't like them. I just don't have a thing for horses. Goats, I love them and have 6 of them. I also love chickens, ducks, and other fowl. And dogs, cats, birds, turtles, etc.