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Adjective: imperviable
  1. Not admitting of passage or capable of being affected
    "a material imperviable to water";
    - impervious

See also: acid-fast, colorfast [US], colourfast [Brit, Cdn], corrosion-resistant, fast, greaseproof, impermeable, ladder-proof, mothproof, moth-resistant, proof, resistant, rot-resistant, runproof, run-resistant, soundproof