'rustproofing' originates from Modern English, formed by combining 'rust' + 'proof' + the gerund/derivational suffix '-ing'.

'rust' comes from Old English 'rūst' meaning 'rust'; 'proof' (used as a combining element meaning 'resistant to') developed through Middle English from Old French/Latin roots related to 'test' or 'prove'. These elements combined in Modern English to form 'rustproof', and adding '-ing' produced 'rustproofing'.

Initially, the components meant 'rust' = oxidation residue and 'proof' = to test/prove; over time 'proof' developed the sense 'resistant to', so the combined form came to mean 'making resistant to rust'.