IPA: /ˈwɔːtərpruːf.nəs/
KK: /ˈwɔːtərpruːf.nəs/
The quality or characteristic of being resistant to water, preventing it from passing through or damaging something.
The waterproofness of the jacket kept me dry during the rain.
Waterproofness → It is formed from "water" (from Old English *wæter*, meaning liquid that falls as rain or is found in rivers and lakes), "proof" (from Old French *prover*, meaning to test or to prove), and "-ness" (a suffix indicating a state or quality). The word "waterproofness" refers to the quality of being impervious to water or resistant to water damage.
Think of the liquid 'water' and the idea of being able to 'prove' something is resistant to it — that's why waterproofness means the quality of being resistant to water.