Etymological Note: Originally applied to the design of seacraft, in which the upper part of the sides of a boat or ship—the topsides—angles upwards from a wider point closer to the waterline.
Citations:
1984 Bill Mitchell Corvette: A Piece of the Action--Impressions of the Marque and the Mystique, 1953-1985 p. 26: We all tried to out-fin each other back in the Fifties, and we let the fat look get out of hand in the Sixties and early Seventies, when the industry got so carried away with turn-under and tumblehome that some of the cars started looking like horseshoe crabs on roller skates. 1990 Jim Kenzie Toronto Star (Canada) (Oct. 6) “Toyota Tercel goes aero for the ’90s” p. SA2 J1: The Tercel’s severe “tumblehome”—the deep curvature of the roof merging into the sides of the car—brings the corners of the cabin threateningly close to your ears. 2007 Phil Patton New York Times (Apr. 1) “Body Language: How to Talk the Designers’ Talk”: With cars, words and metal share territory: each brand’s vocabulary of shapes is collectively known as its design language. The beltline divides the greenhouse, or glassed-in upper body, from the portion that extends down from the window sills.…The angle of the windshield is known as its rake—an extreme tilt is said to be fast—while the inward angle of the side greenhouse windows is called the tumblehome.