"Bulk up" means to increase muscle mass and size through exercise and diet . It's like when someone lifts weights and eats protein-rich foods to build stronger muscles. Bulking up involves a combination of strength training and consuming extra calories to support muscle growth. People bulk up to improve their physical strength and appearance. It's common among athletes, bodybuilders , and fitness enthusiasts who want to enhance their muscle size and power. So, "bulking up" is about intentionally increasing muscle size and mass through targeted exercise and diet ary habits.