airtight

the compound word 'airtight' is a modern english formation that transparently combines two native elements. 'air' originates from greek aēr, referring to the lower atmosphere, which passed into latin and then old french before entering middle english. 'tight' derives from old norse þéttr, meaning 'dense, watertight', which influenced middle english. when combined in the 18th century, the literal sense 'so tightly sealed as to prevent the passage of air' emerged directly from the sum of its parts. this concrete meaning was later extended metaphorically to arguments or alibis that are impervious to criticism, completing a logical journey from a physical seal to an abstract quality of being flawless or unassailable.

Structure: air + tight air (from middle english, old french, latin aer, greek aēr; meaning 'the invisible gaseous substance surrounding the earth') + tight (from middle english, old english -tht, related to thight meaning 'dense, impermeable'; meaning 'firmly fixed or sealed').