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.
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         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.
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The Collaborative International Dictionary of English v.0.48:

Impair \Im*pair"\, v. t.
   To grow worse; to deteriorate. --Milton.
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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.]
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The Collaborative International Dictionary of English v.0.48:

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