Sealed so completely that air cannot enter or escape, or metaphorically, a plan or argument with no flaws or weaknesses.
Compound of 'air' (from Old English 'ær') and 'tight' (from Old English 'tihte'). First used literally in the 19th century for containers, then metaphorically by the early 20th century.
Medieval knights' armor wasn't actually airtight, but NASA spacesuits must be—the challenge of creating airtight seals in space led to technologies now used in everything from dishwashers to surgical equipment.
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