sacristy
Concise Oxford English Dictionary © 2008 Oxford University Press:
sacristy /ˈsakrɪsti/
▶noun (pl. sacristies) a room in a church where a priest prepares for a service, and where vestments and other things used in worship are kept.
– origin ME: from Fr. sacristie, from med. L. sacristia, based on L. sacer, sacr- ‘sacred’.
'sacristy' also found in these Oxford entries:

