The Collaborative International Dictionary of English v.0.48:

Sacristy \Sac"ris*ty\, n.; pl. Sacristies. [F. sacristie, LL.
   sacristia, fr. L. sacer. See Sacred.]
   An apartment in a church where the sacred utensils,
   vestments, etc., are kept; a vestry.
   [1913 Webster]