A housecoat or a duster, was worn when women in the 1940s were cleaning their house in order to protect what they were wearing for the day. Housecoats, long buttoned robes of light fabric ( but could also be made of warmer material ) became popular since they covered more than aprons or pinafores. A housecoat was an informal evening wear, usually roomy with buttons or a zip in front. Lengths were knee length or longer. Another way to use it was as robe over ones underwear, for instance when you needed to do hair or put on makeup.