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]

Moby Thesaurus II by Grady Ward, 1.0:

123 Moby Thesaurus words for "disadvantage":
   abuse, ado, afflict, aggrieve, annoyance, anxiety, bar, befoul,
   besetment, bewitch, blight, blocking, bother, burden, burthen,
   can of worms, cargo, charge, condemn, corrupt, cross, crucify,
   cumbrance, curse, damage, deadweight, defect, defile, deprave,
   deprivation, despoil, destroy, detriment, difficulty, disability,
   disaccommodate, discommode, disoblige, disserve, disservice,
   distress, do a mischief, do evil, do ill, do wrong, do wrong by,
   doom, drawback, embarrassment, encumbrance, envenom, evil, fault,
   flaw, freight, get into trouble, great ado, hamper, handicap,
   harass, harm, headache, hex, hindrance, hurt, impair, impairment,
   impediment, impedimenta, impose upon, imposition, incommode,
   inconvenience, infect, injure, injury, jinx, liability, load, loss,
   loss of ground, lumber, maltreat, matter, menace, mischief,
   mistreat, molest, obstacle, obstruction, onus, outrage, pack,
   peck of troubles, penalty, persecute, play havoc with,
   play hob with, poison, pollute, prejudice, problem, put out,
   put to inconvenience, put to trouble, savage, scathe,
   sea of troubles, shortcoming, step backward, taint, threaten,
   torment, torture, trouble, violate, weakness, weight,
   white elephant, worry, wound, wreak havoc on, wrong