Lots of people have heard of a tonneau cover, a removable cloth or vinyl cover for an open-topped car, often that can be unzipped/unsnapped just over the driver's seat and otherwise left on, or alternatively a cover for the cargo space of a pickup truck, station wagon or SUV. But what's a tonneau, anyway?
In French, a tonneau is a barrel, specifically a 900 litre barrel, the largest size of wine barrel in general use. French wine in bulk is still sold in units of that size.
A number of things whose shape is somewhat (arguably) barrel-like are described by this word, and some of them have carried over into English.
In the automotive field, specifically of early times and thus derived from an earlier, coachbuilding usage, a tonneau is an open-topped car, lacking a roof in any way, that possesses a bench front seat and a semi-circular rear seat, incorporating a rear access door built into the seat (normally, the center part of the seat cushion folds up against the door, which then swings outward). The sides of the car curve smoothly around to the rear, and thus the rear compartment is a little reminiscent of an open barrel or half-barrel, close to round, tapering towards the bottom, and surrounded on all sides.
Thus, originally, a tonneau cover was a cover for a car like this, to place over it while leaving it outside so the barrel-like rear compartment wouldn't fill with water.
Wristwatches can also be described as a 'tonneau' - in this case it refers to the watches that have a rectangular face but with the sides bowed out, so that the face is the shape of a barrel seen from its side.