Source : Webster's Revised Unabridged Dictionary (1913)

Incompressible \In`com*press"i*ble\, a. [Pref. in- not +
   compressible: cf. F. incompressible.]
   Not compressible; incapable of being reduced by force or
   pressure into a smaller compass or volume; resisting
   compression; as, many liquids and solids appear to be almost
   incompressible. -- {In`com*press"i*ble*ness}, n.

Source : WordNet®

incompressible
     adj : impossible to compress; resisting compression; "mounds of
           incompressible garbage" [ant: {compressible}]