the rear seating compartment of a motor car
a cover used to protect the seats in an open-topped vehicle
French tonneau, literally ‘tun’, from Old French tonel: see tunnel1. The word orig denoted a unit of capacity for wine (equal to 900l); in the early 20th cent. it was applied to the rounded rear part of a car, thought to resemble a barrel, hence the rear compartment, whatever its shape. Sense 2 is short for tonneau cover