A term used especially in the Catholic and Episcopal Churches to describe an unordained individual so devoted to his Church that he is seldom seen to leave. He can often be observed meticulously arranging the fair linens, compulsively rearranging the altar candles in descending order of height, and maintaining the integrity of the reserve sacrament at all hours of the night. Outside of their eccleastical duties, Sacristy rats have a reputation for being heavy drinkers, particularly in the Episcopal Church.
"Look, Richard's rearranging the linens again. He just did that a few minutes ago."
"Dude's a Sacristy Rat, yo."