noun
loungewear; indoor clothes; comfy home clothes
Casual, comfortable clothing worn at home; distinct from pajamas or going-out clothes.
帰宅したらすぐに部屋着に着替える。
I change into loungewear as soon as I get home.
休日は一日中部屋着で過ごす。
On my days off, I spend the whole day in my comfy home clothes.
普段着 is everyday casual wear that can be worn outside, whereas 部屋着 is strictly for indoors.
Compound of 部屋 (room) and 着 (clothing). A straightforward modern coinage for indoor loungewear.