The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]