Lewis & Short

ergastŭlārĭus, a, um, adj. [ergastulum], of or belonging to a workhouse or house of correction (late Lat.).

  1. I. Adj.: servus, Amm. 14, 11 fin.
    Also: in tenebris ergastularibus, Sid. Ep. 7, 9.
  2. II. Subst.: ergastularii, ōrum, m. (sc. servi), overseers of a penitentiary, Col. 1, 8, 17.