Emily Dickinson Lexicon

IM-PER'VI-OUS, a. [L. impervius; in and pervius, passable; per and via, way.]

1. Not to be penetrated or passed through; impenetrable; as, an impervious gulf; an impervious forest.
2. Not penetrable; not to be pierced by a pointed instrument; as, an impervious shield.
3. Not penetrable by light; not permeable to fluids. Glass is pervious to light, but impervious to water. Paper is impervious to light. In the latter sense only, impervious is synonymous with impermeable.