The Collaborative International Dictionary of English v.0.48:

Impair \Im*pair"\, v. t. [imp. & p. p. Impaired; p. pr. & vb.
   n. Impairing.] [Written also empair.] [OE. empeiren,
   enpeiren, OF. empeirier, empirier, F. empirer, LL.
   impejorare; L. pref. im- in + pejorare to make worse, fr.
   pejor worse. Cf. Appair.]
   To make worse; to diminish in quantity, value, excellence, or
   strength; to deteriorate; as, to impair health, character,
   the mind, value.
   [1913 Webster]

         Time sensibly all things impairs.        --Roscommon.
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         In years he seemed, but not impaired by years. --Pope.

   Syn: To diminish; decrease; injure; weaken; enfeeble;
        debilitate; reduce; debase; deteriorate.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Impair \Im*pair"\, v. t.
   To grow worse; to deteriorate. --Milton.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Impair \Im"pair\, a. [F. impair uneven, L. impar; im- not + par
   equal.]
   Not fit or appropriate. [Obs.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Impair \Im*pair"\, n.
   Diminution; injury. [Obs.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

impair
    v 1: make worse or less effective; "His vision was impaired"
    2: make imperfect; "nothing marred her beauty" [syn: mar,
       impair, spoil, deflower, vitiate]

Moby Thesaurus II by Grady Ward, 1.0:

109 Moby Thesaurus words for "impair":
   abate, abrade, abstract, abuse, afflict, aggravate, aggrieve, bate,
   befoul, bewitch, blemish, blight, condemn, corrupt, cripple,
   crucify, curse, curtail, damage, decrease, deduct, defile, deprave,
   depreciate, derogate, despoil, destroy, deteriorate, detract,
   dilapidate, diminish, disadvantage, disparage, disserve, distress,
   do a mischief, do evil, do ill, do wrong, do wrong by, doom, drain,
   eat away, embitter, endamage, envenom, erode, exacerbate, extract,
   file away, get into trouble, harass, harm, hex, hurt, infect,
   injure, irritate, jinx, leach, lessen, make worse, maltreat, mar,
   menace, mistreat, molest, outrage, persecute, play havoc with,
   play hob with, poison, pollute, prejudice, purify, put back,
   reduce, refine, remove, retrench, rub away, ruin, sap, savage,
   scathe, shorten, spoil, subduct, subtract, taint, take away,
   take from, tarnish, thin, thin out, threaten, torment, torture,
   undermine, violate, vitiate, weaken, wear away, weed, withdraw,
   worsen, wound, wreak havoc on, wrong