Not to be coerced; incapable of being compelled or forced.
2.
(Physics) Not capable of being reduced to the form of a liquid by pressure; said of any gas above its critical temperature.
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.
... PENETRABILITY OF MATTER: I mean chemistry. In fact, how does what is called chemical composition differ from penetration?[5]. . . . In short, we know matter only through its forms; of its substance we know nothing. How, then, is it possible to affirm the reality of an invisible, impalpable, incoercible being, ever changing, ever vanishing, impenetrable to thought alone, to which it exhibits only its disguises? Materialist! I permit you to testify to the reality of your sensations; as to what occasions ... — The Philosophy of Misery • Joseph-Pierre Proudhon
...
