powder room

Short definition from Wiktionary

Noun

1. 1
   Synonym of ladies' room: a lavatory intended for use by women.
2. 2
   Synonym of half bath: a small room with a toilet and sink but no bathtub or shower.
3. 3
   A room used to store gunpowder, particularly (historical) on a ship equipped with cannon.
4. 4
   A room used to powder hair and wigs.