Loo is a form of English slang commonly spoken in the 20th century, and was used in reference to a toilet. While showing James T. Kirk and Spock around their apartment in 1930, Edith Keeler told Kirk that the "loo" was just down the hall. When Kirk didn't comprehend her meaning, Keeler apologized and said that her English was shining through. (TOS novel: Crucible: The Fire and the Rose)