The Collaborative International Dictionary of English v.0.48:

Workhouse \Work"house`\, n.; pl. Workhouses. [AS.
   weorch[=u]s.]
   [1913 Webster]
   1. A house where any manufacture is carried on; a workshop.
      [1913 Webster]

   2. A house in which idle and vicious persons are confined to
      labor.
      [1913 Webster]

   3. A house where the town poor are maintained at public
      expense, and provided with labor; a poorhouse.
      [1913 Webster]

WordNet (r) 3.0 (2006):

workhouse
    n 1: a poorhouse where able-bodied poor are compelled to labor
    2: a county jail that holds prisoners for periods up to 18
       months

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "workhouse":
   agency, almshouse, asylum, atelier, barbershop, beauty parlor,
   beauty shop, bench, butcher shop, company, concern, corporation,
   desk, establishment, facility, firm, foster home, halfway house,
   home, hospice, hospitium, house, installation, institution, loft,
   nursing home, organization, orphanage, parlor, poor farm,
   poorhouse, rest home, retreat, shop, studio, sweatshop, work site,
   work space, workbench, working space, workplace, workroom,
   workshop, worktable



Bouvier's Law Dictionary, Revised 6th Ed (1856):

WORKHOUSE. A prison where prisoners are kept in employment; a penitentiary.
A house provided where the poor are taken care of, and kept in employment.