Bulk up Definition 1 of 2Pro Tip 1/3Physical Transformation Used to express a change in someone’s physical appearance, often through exercise. After joining the gym, Mia bulked up significantly. Pro Tip 2/3Sports Context Commonly used in sports to describe gaining muscle for better performance. The athletes are bulking up to prepare for the wrestling championships. Pro Tip 3/3Personal Goals Indicates an intentional effort, often for aesthetic reasons or personal achievement. He's bulking up to feel more confident at the beach this summer.