Firm up Definition 1 of 2Pro Tip 1/3Physical Strength Used when referring to the physical body gaining strength or muscle tone. Even after the injury, you can still firm up your core with exercise. Pro Tip 2/3Progressive Change Describes a gradual improvement or development in something over time. Your pasta-making skills will firm up as you practice more. Pro Tip 3/3Not Instant Emphasizes that firming up is a process, which requires time and persistence. With consistent effort, your baking results will firm up.