The word is a modern English compound formed by combining three distinct morphemes. The first, "water", traces its lineage directly from the Proto-Indo-European term for the essential liquid. The second, "proof", has a journey from Latin concepts of testing and proving, evolving in English to denote resistance, as in something that has been tested against a force. The suffix "-ing" is a native English element that nominalizes the adjective "waterproof". Thus, the literal construction "water-resistant-making" logically yields the modern abstract meanings of either the process of applying such resistance or the material that provides it.

Structure: water (liquid) + proof (resistant) + ing (noun suffix) water: Old English 'wæter', from Proto-Germanic watōr, from Proto-Indo-European wódr̥. The core element representing the liquid H₂O. proof: Middle English, from Old French 'preuve', from Late Latin 'proba' (a test), from Latin 'probare' (to test, prove). In this compound, it functions as a suffix meaning "resistant to, impervious to". ing: Old English '-ing', '-ung', a suffix forming nouns from verbs, indicating an action, process, or material resulting from an action.