a. [ Pref. im- not + permeable: cf. F. imperméable, L. impermeabilis. ] Not permeable; not permitting passage, as of a fluid. through its substance; impervious; impenetrable; as, India rubber is impermeable to water and to air. -- Im*per"me*a*ble*ness, n. -- Im*per"me*a*bly, adv. [ 1913 Webster ]