From Swimming; Acronym for: Get In Shut The Fuck Up And Swim. A term used to describe a long swimming workout or a specific long challenging swimming set with few breaks or rest periods. May also be used to describe a training set given by a coach to deal with athletes who are slow to begin their warm up.
"Today's practice is total GISTFUAS, coach has us swimming 4,000 meters pretty much straight through as our warm-up." or "Wow, 5 x 1,000s to start the practice? What a GISTFUAS set."