The British word for the toilet, “loo”, derives from the French “guardez l’eau”, meaning “watch out for the water”. This comes from the fact that, in medieval Europe, people simply threw the contents of their chamber pots out the window onto the streets. Before throwing the waste out the window, they’d yell “Guardez l’eau!” The term “guardez l’eau” first came to English as “gardy-loo” and then shortened to “loo”, which eventually came to mean the toilet itself.
The term “toilet” itself comes from the French “toilette”, which meant “dressing room”. This “toilette” in turn derived from the French “toile”, meaning “cloth”; specifically, referring to the cloth draped over someone’s shoulders while their hair was being groomed. During the 17th century, the toilet was simply the process of getting dressed, fixing your hair, and applying make-up and the like, more or less grooming one’s self. This gradually began to refer to the items around where someone was groomed, such as the table, powder bottles, and other items. Around the 1800s in America, this term began being used to refer to both the room itself where people got dressed and ready for the day, as well as the device itself now most commonly known as the toilet.
The term “lavatory” also derives from the Latin “lavare”, although this time through the Middle Latin variation “lavatorium”, meaning “washbasin”. This popped up in English around the late 19th century.