Is it ensure, insure, or assure?
late 14c., probably from Anglo-French *vesterie, from Old French vestiaire "room for vestments," from Latin vestarium "wardrobe," noun use of neuter of vestiarius (adj.) "of clothes," from vestis "garment" (see vest (v.)).
in architecture, room in a Christian church in which vestments and sacred objects used in the services are stored and in which the clergy and sometimes the altar boys and the choir members put on their robes. In the early Christian church, two rooms beside the apse, the diaconicon and the prothesis, were used for these purposes.