gut-buster
/ˈɡʌtˌbʌstə/informal
noun
1Something which hurts the stomach, literally or figuratively; something involving or requiring a large amount of effort or energy.
2An exercise, or piece of exercise equipment, designed to help weight loss or strengthen the abdominal muscles.
Origin
1920s; earliest use found in William Faulkner (1897–1962), short-story writer and novelist. From gut + -buster, after to bust a gut.