The Collaborative International Dictionary of English v.0.48:

Tonneau \Ton`neau"\, n.; pl. Tonneaux. [F.]
   1. In France, a light-wheeled vehicle with square or rounded
      body and rear entrance.
      [Webster 1913 Suppl.]

   2. (Automobiles) Orig., the after part of the body with
      entrance at the rear (as in vehicle in def. 1); now, one
      with sides closing in the seat or seats and entered by a
      door usually at the side, also, the entire body of an
      automobile having such an after part.
      [Webster 1913 Suppl.]

   3. same as Tonne.
      [Webster 1913 Suppl.]