Etymology
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impermeable (adj.)
1690s, from French imperméable or directly from Late Latin impermeabilis, from assimilated form of in- "not, opposite of" (see in- (1)) + permeabilis (see permeable). Related: Impermeability.
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airtight (adj.)
also air-tight, "impermeable to air," 1760, from air (n.1) + tight. Figurative sense of "incontrovertible" (of arguments, alibis, etc.) is from 1929.
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