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.
      [1913 Webster]

            Infirm of purpose!                    --Shak.
      [1913 Webster]

   3. Not solid or stable; insecure; precarious.
      [1913 Webster]

            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

Moby Thesaurus II by Grady Ward, 1.0:

202 Moby Thesaurus words for "infirm":
   Adamic, abulic, adrift, afloat, afraid, ailing, alternating,
   amorphous, anile, backsliding, cachectic, capricious, carnal,
   changeable, changeful, cowardly, crabbed, crippled, crumbling,
   dangerous, debilitated, decrepit, desultory, deviable,
   disintegrating, dizzy, doddered, doddering, doddery, drained,
   eccentric, enervated, enfeebled, erratic, erring, exhausted,
   failing, faint, fainthearted, fallen, faltering, fast and loose,
   feeble, feebleminded, fickle, fitful, fleshly, flickering, flighty,
   flimsy, flitting, fluctuating, fossilized, fragile, frail,
   freakish, gerontal, gerontic, giddy, hazardous, healthless, ill,
   impetuous, impulsive, impure, in poor health, inconsistent,
   inconstant, indecisive, indisposed, insecure, insubstantial,
   invalid, invertebrate, irregular, irresolute, irresponsible, lame,
   languishing, lapsed, mazy, mercurial, moody, moribund, mossbacked,
   moth-eaten, mummylike, of easy virtue, on the decline, pale,
   palsied, papery-skinned, peaked, peaky, peccable, perilous,
   pliable, poor, poorish, postlapsarian, precarious, prodigal,
   provisional, rambling, ravaged with age, recidivist, recidivistic,
   reduced, reduced in health, restless, rickety, risky, rotten,
   rotten at, roving, run to seed, run-down, rusty, scatterbrained,
   senile, shaky, shapeless, shifting, shifty, shriveled, shuffling,
   sick, sickly, slippery, soft, spasmodic, spineless,
   stricken in years, temporary, tentative, ticklish, timeworn,
   tottering, tottery, treacherous, unaccountable, unangelic,
   uncertain, unchaste, unclean, uncontrolled, undependable,
   undisciplined, unfaithworthy, unfirm, unfixed, ungodly, ungood,
   unhealthy, unpredictable, unreliable, unrestrained, unrighteous,
   unsaintly, unsettled, unsolid, unsound, unstable,
   unstable as water, unstaid, unsteadfast, unsteady, unsturdy,
   unsubstantial, unsure, untrustworthy, unvirtuous, unwell,
   vacillating, vagrant, valetudinarian, valetudinary, variable,
   vicissitudinary, vicissitudinous, virtueless, volatile, wandering,
   wanton, wasted, wavering, wavery, wavy, wayward, weak, weak-kneed,
   weak-minded, weak-willed, weakened, weakly, whimsical, wishy-washy,
   with low resistance, withered, wizened, wobbling, wobbly



Bouvier's Law Dictionary, Revised 6th Ed (1856):

INFIRM. Weak, feeble.
     2. When a witness is infirm to an extent likely to destroy his life, or
to prevent his attendance at the trial, his testimony de bene esge may be
taken at any age. 1 P. Will. 117; see Aged witness.; Going witness.