The Collaborative International Dictionary of English v.0.48:

Carboxide \Car*box"ide\, n. [Carbon + oxide.] (Chem.)
   A compound of carbon and oxygen, as carbonyl, with some
   element or radical; as, potassium carboxide.
   [1913 Webster]

   Potassium carboxide, a grayish explosive crystalline
      compound, C6O6K, obtained by passing carbon monoxide
      over heated potassium.
      [1913 Webster]