'saltproof' originates from Modern English, specifically the compound of Old English 'sealt' and the element 'proof' (from Middle English/Old French 'preuve'), where 'sealt' meant 'salt' and 'proof' meant 'resistant' or 'tested against'.

'saltproof' formed in modern usage by combining 'salt' + 'proof' in the same way as compounds like 'waterproof' or 'bulletproof'; 'proof' developed in Middle English from Old French 'preuve' (from Latin 'probare') meaning 'test' and came to be used as a suffix indicating resistance.

Initially the components referred to 'salt' and the idea of being 'tested' or 'protected against' (from 'proof'); over time the compound came to be used straightforwardly to mean 'not affected by salt or saltwater' in physical/material contexts.