The Collaborative International Dictionary of English v.0.48:

Incoercible \In`co*er"ci*ble\, a. [Pref. in- not + coercible:
   cf. F. incoercible.]
   [1913 Webster]
   1. Not to be coerced; incapable of being compelled or forced.
      [1913 Webster]

   2. (Physics) Not capable of being reduced to the form of a
      liquid by pressure; -- said of any gas above its critical
      temperature.
      [1913 Webster +PJC]

   3. (Physics) That can note be confined in, or excluded from,
      vessels, like ordinary fluids, gases, etc.; -- said of the
      imponderable fluids, heat, light, electricity, etc.
      [1913 Webster]