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.
   [1913 Webster]

         In years he seemed, but not impaired by years. --Pope.

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

WordNet (r) 3.0 (2006):

impaired
    adj 1: diminished in strength, quality, or utility; "impaired
           eyesight" [ant: unimpaired]
    2: mentally or physically unfit [syn: afflicted, impaired]