“Fats folks falling down stairs” refers to a particular sort of bodily comedy during which an obese individual loses their steadiness and falls down a flight of stairs. This comedic trope is usually used to elicit laughter from audiences, because it performs on the sudden and slapstick nature of the state of affairs.
Using “fats folks falling down stairs” as a comedic gadget has an extended historical past, relationship again to early types of leisure comparable to vaudeville and burlesque. In these performances, comedians would usually use their physicality to create humor, and falling down stairs was a typical means to take action.