Source : Webster's Revised Unabridged Dictionary (1913)

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.

         Time sensibly all things impairs.        --Roscommon.

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

   Syn: To diminish; decrease; injure; weaken; enfeeble;
        debilitate; reduce; debase; deteriorate.

Source : WordNet®

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