'gas-tight' is a compound formed in English from 'gas' + 'tight'. 'Gas' entered English in the 17th century from Dutch 'gas' (coined in early modern science), and 'tight' comes from Old English words meaning 'firm, closely drawn'.

'gas' was adopted into English from Dutch in the 1600s (used in early chemistry and natural philosophy). 'Tight' comes from Old English roots and continued into Modern English; the compound 'gas-tight' developed in modern English usage to describe something that keeps gas in or out.

Originally the elements 'gas' and 'tight' had separate meanings ('gas' for the substance and 'tight' for close-fitting). Over time, the compound came to specifically mean 'impermeable to gas' or 'providing a seal against gas leakage'.