The Collaborative International Dictionary of English v.0.48:

Incombustible \In`com*bus"ti*ble\, a. [Pref. in- not +
   combustible: cf. F. incombustible.]
   Not combustible; not capable of being burned, decomposed, or
   consumed by fire; uninflammable; as, asbestus is an
   incombustible substance; carbon dioxide is an incombustible
   gas.
   [1913 Webster]

   Incombustible cloth, a tissue of amianthus or asbestus;
      also, a fabric imbued with an incombustible substance. --
      In`com*bus"ti*ble*ness, n. -- In`com*bus"ti*bly, adv.
      [1913 Webster]

WordNet (r) 3.0 (2006):

incombustible
    adj 1: not capable of igniting and burning [syn:
           noncombustible, incombustible] [ant: combustible]