The Collaborative International Dictionary of English v.0.48:

Infirm \In*firm"\ ([i^]n*f[~e]rm"), a. [L. infirmus: cf. F.
   infirme. See In- not, and Firm, a.]
   1. Not firm or sound; weak; feeble; as, an infirm body; an
      infirm constitution.
      [1913 Webster]

            A poor, infirm, weak, and despised old man. --Shak.
      [1913 Webster]

   2. Weak of mind or will; irresolute; vacillating. "An infirm
      judgment." --Burke.
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            Infirm of purpose!                    --Shak.
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   3. Not solid or stable; insecure; precarious.
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            He who fixes on false principles treads or infirm
            ground.                               --South.

   Syn: Debilitated; sickly; feeble; decrepit; weak; enfeebled;
        irresolute; vacillating; imbecile.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Infirm \In*firm"\, v. t. [L. infirmare : cf. F. infirmer.]
   To weaken; to enfeeble. [Obs.] --Sir W. Raleigh.
   [1913 Webster]

WordNet (r) 3.0 (2006):

infirm
    adj 1: lacking bodily or muscular strength or vitality; "a
           feeble old woman"; "her body looked sapless" [syn:
           decrepit, debile, feeble, infirm, rickety,
           sapless, weak, weakly]
    2: lacking firmness of will or character or purpose; "infirm of
       purpose; give me the daggers" - Shakespeare