It sounds logical that you would perform a "lavation" in a "lavatory," doesn't it? And it is logical: both words come from Latin "lavare," meaning, appropriately, "to wash." English picked up a few other words from this root as well. In medicine, the therapeutic washing out of an organ is "lavage." There is also "lavabo" (in Latin, literally, "I shall wash"), which in English can refer to a ceremony at Mass in which the celebrant washes his hands, to the basin used in this religious ceremony, or to other kinds of basins. Even the word "lavish," via a Middle French word for a downpour of rain, comes to us from "lavare."