- Impenetrable - Invincible - Invulnerable - Not to be overthrown - Unassailable - Not to be stormed, or taken by assault; incapable of
being subdued; able to resist attack; unconquerable; as, an impregnable
fortress; impregnable virtue. - Capable of being impregnated, as the egg of an animal,
or the ovule of a plant.
- Impenetrable - Not permeable; not permitting passage, as of a fluid.
through its substance; impervious; impenetrable; as, India rubber is
impermeable to water and to air.