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]

Moby Thesaurus II by Grady Ward, 1.0:

76 Moby Thesaurus words for "impaired":
   adulterated, aggravated, blemished, broken, burned, burst, busted,
   checked, chipped, cracked, crazed, cut, damaged, defective,
   deficient, deteriorated, embittered, erroneous, exacerbated,
   fallible, faulty, flawed, found wanting, harmed, hurt, immature,
   imperfect, imprecise, impure, in bits, in pieces, in shards,
   inaccurate, inadequate, incomplete, inexact, injured, irritated,
   lacerated, lacking, makeshift, mangled, marred, mediocre, mixed,
   mutilated, not perfect, off, partial, patchy, rent, ruptured,
   scalded, scorched, shattered, short, sketchy, slashed, slit,
   smashed, split, spoiled, sprung, the worse for, torn, undeveloped,
   uneven, unfinished, unperfected, unsound, unthorough, wanting,
   weakened, worse, worse off, worsened