'waterproofer' originates from Modern English, formed by compounding 'waterproof' with the agentive suffix '-er' to mean 'one that makes or renders something waterproof.'

'waterproof' itself is a compound of Old English 'wæter' (water) and Middle English/Old French-influenced 'proof' (from Old French 'preuve'/'pruef'), and the agentive '-er' was later added in Modern English to form 'waterproofer.'

Initially, 'proof' meant 'to test' or 'to demonstrate' but developed a sense of 'resistant to' in compounds (e.g., 'waterproof' = 'resistant to water'); 'waterproofer' therefore came to mean either an agent (product) that makes something resistant to water or a person who applies such treatment.