It stands for “Water Closet.”
The phrase dates back to the 19th century, when indoor plumbing was still a groundbreaking innovation. Before modern bathrooms, people relied on outhouses, chamber pots, or shared facilities. When flushing toilets began appearing inside homes, they were often installed in small, enclosed rooms separate from the main bathing area. These rooms were literally closets that contained water-powered toilets — hence the term water closet.
