The Collaborative International Dictionary of English v.0.48:

Ancillary \An"cil*la*ry\, a. [L. ancillaris, fr. ancilla a
   female servant.]
   Subservient or subordinate, like a handmaid; auxiliary.
   [1913 Webster]

         The Convocation of York seems to have been always
         considered as inferior, and even ancillary, to the
         greater province.                        --Hallam.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ancillary
    adj 1: furnishing added support; "an ancillary pump"; "an
           adjuvant discipline to forms of mysticism"; "The mind and
           emotions are auxiliary to each other" [syn: accessory,
           adjunct, ancillary, adjuvant, appurtenant,
           auxiliary]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "ancillary":
   accessory, accompanying, additional, adjuvant, another,
   appurtenant, assistant, assisting, attendant, attending, auxiliary,
   coincident, collateral, contributory, extra, farther, fostering,
   fresh, further, helping, incident, instrumental, ministerial,
   ministering, ministrant, more, new, nurtural, nutricial, other,
   plus, satellite, serving, spare, subservient, subsidiary,
   supernumerary, supplemental, supplementary, surplus, ulterior



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

ANCILLARY. That which is subordinate on, or is. subordinate to, some other
decision. Encyc. Lond. 1