In a manner that is completely sealed so that air cannot escape or enter.
From 'airtight' (a 19th-century compound of 'air' + 'tight') plus the adverbial suffix -ly. The adjective 'airtight' originally described stoves and containers.
When you seal something 'airtightly,' you're creating a miniature world with its own atmosphere—this matters hugely in space travel, food preservation, and even in old science fiction where 'airtightly sealed' rooms became a trope for danger or mystery.
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