The Collaborative International Dictionary of English v.0.48:

Disadvantage \Dis`ad*van"tage\, v. t. [Cf. F. d['e]savantager.]
   To injure the interest of; to be detrimental to.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Disadvantage \Dis`ad*van"tage\ (?; 48, 61), n. [Cf. F.
   d['e]savantage.]
   1. Deprivation of advantage; unfavorable or prejudicial
      quality, condition, circumstance, or the like; that which
      hinders success, or causes loss or injury.
      [1913 Webster]

            I was brought here under the disadvantage of being
            unknown by sight to any of you.       --Burke.
      [1913 Webster]

            Abandoned by their great patron, the faction
            henceforward acted at disadvantage.   --Palfrey.
      [1913 Webster]

   2. Loss; detriment; hindrance; prejudice to interest, fame,
      credit, profit, or other good.
      [1913 Webster]

            They would throw a construction on his conduct, to
            his disadvantage before the public.   --Bancroft.

   Syn: Detriment; injury; hurt; loss; damage.
        [1913 Webster]

WordNet (r) 3.0 (2006):

disadvantage
    n 1: the quality of having an inferior or less favorable
         position [ant: advantage, vantage]
    v 1: put at a disadvantage; hinder, harm; "This rule clearly
         disadvantages me" [syn: disadvantage, disfavor,
         disfavour] [ant: advantage]