I've come across the word ablution several times in the past day. It seemed familiar, but I couldn't place why. I looked it up in Merriam-Webster.com, which gave the definition: "1 a: the washing of one's body or part of it (as in a religious rite) b: the act or action of bathing." OK, now I knew what it meant. (I'd been seeing it used just in the sense of bathing, not in a religious sense.) That didn't explain why I thought it seemed familiar beyond just having read it before. So I checked the OED, and found it, in two parts, definitions 1e and 2a:
1. e. In Rom. and Angl. Churches: the washing of chalice and paten after the celebration. In Rom. Ch., the washing of the priest's hands before assuming the sacred vestments, and during the celebration. 2. a. The water etc. with which anything has been washed; spec. in Catholic Ritual, the wine and water used to rinse the chalice, and wash the fingers of the celebrant after the communion.
Oh, right. I was an altar boy (altar server after they started letting girls participate) in grade school. One of the altar boys' many jobs was to bring the water after Communion for the priest to wash out the chalice and then wash his fingers. We didn't call it ablution among ourselves (I don't think we really called it anything), but I think I knew the word from that.
It comes from the Latin abluere, meaning to wash off.