noun
dressing gown; makeup robe
A robe or gown worn while doing one's makeup or getting ready, often at a vanity or dressing table.
彼女は化粧着を羽織って鏡の前に座った。
She put on a dressing gown and sat in front of the mirror.
部屋着 is general loungewear or indoor clothing, not specifically associated with makeup application.
Compound of 化粧 (keshō, 'makeup') and 着 (gi, 'clothing'). The reading けしょうぎ follows standard rendaku voicing of 着.