'leakproof' originates from English, specifically a compound of 'leak' + 'proof', where 'leak' comes from Old English/Old Norse roots meaning 'a leak or escape of liquid/gas' and 'proof' (from Old French 'proef', ultimately Latin 'probare') carried the sense 'tested against' or 'resistant to'.

'leakproof' developed as part of a productivity of compounds with 'proof' (as in 'waterproof', 'fireproof'); earlier phrases like 'proof against leaks' and hyphenated forms ('leak-proof') gradually gave the modern single-word 'leakproof' and later the verb use 'to leakproof'.

Initially it simply conveyed 'resistant to leaks' and that core meaning has been retained; over time the term also came to be used as a verb meaning 'to make resistant to leaking'.