IPA: /ˈbæθˌwɔːtə/
KK: /ˈbæθˌwɔːtər/
The water that is used in a bathtub for bathing.
After the bath, I let the bathwater drain away.
Bathwater → The word is formed from "bath" (from Old English *baþ*, meaning a washing or immersion) and "water" (from Old English *wæter*, meaning water). The term "bathwater" refers to the water used for bathing or washing oneself.
Think of 'washing' ('bath') and 'water' together — bathwater is simply the water used for washing.