The Collaborative International Dictionary of English v.0.48:

Decrepit \De*crep"it\, a. [L. decrepitus, perhaps orig., noised
   out, noiseless, applied to old people, who creep about
   quietly; de- + crepare to make a noise, rattle: cf. F.
   d['e]cr['e]pit. See Crepitate.]
   Broken down with age; wasted and enfeebled by the infirmities
   of old age; feeble; worn out. "Beggary or decrepit age."
   --Milton.
   [1913 Webster]

         Already decrepit with premature old age. --Motley.
   [1913 Webster]

   Note: Sometimes incorrectly written decrepid.
         [1913 Webster]

WordNet (r) 3.0 (2006):

decrepit
    adj 1: worn and broken down by hard use; "a creaky shack"; "a
           decrepit bus...its seats held together with friction
           tape"; "a flea-bitten sofa"; "a run-down neighborhood";
           "a woebegone old shack" [syn: creaky, decrepit,
           derelict, flea-bitten, run-down, woebegone]
    2: lacking bodily or muscular strength or vitality; "a feeble
       old woman"; "her body looked sapless" [syn: decrepit,
       debile, feeble, infirm, rickety, sapless, weak,
       weakly]

Moby Thesaurus II by Grady Ward, 1.0:

109 Moby Thesaurus words for "decrepit":
   aged, ancient, anile, antiquated, battered, beat-up, beaten up,
   bedraggled, broken-down, cast-off, childish, childlike, crabbed,
   creaking, creaky, crippled, crumbling, damaged, debilitated,
   decayed, decaying, derelict, deteriorated, dilapidated, disabled,
   disintegrating, doddered, doddering, doddery, doited, doting,
   down-at-heel, elderly, enervated, enfeebled, feeble, flimsy,
   fossilized, fragile, frail, gaga, gerontal, gerontic, haggard,
   impaired, in ruins, incapacitated, infirm, injured, insubstantial,
   marred, mossbacked, moth-eaten, mummylike, old, out of shape,
   palsied, papery-skinned, poor, poorish, quavering, ragged,
   ramshackle, ravaged with age, rickety, rotten, rotten at, ruined,
   ruinous, run to seed, run-down, rusty, seedy, senescent, senile,
   shabby, shaking, shaky, shriveled, slipshod, sloppy, slummy,
   spoiled, stricken in years, superannuated, tacky, threadbare,
   timeworn, tired, tottering, tottery, tumbledown, unfirm, unfit,
   unkempt, unsolid, unsound, unstable, unsturdy, unsubstantial, used,
   wasted, weak, weakened, weakly, withered, wizened, worn,
   worn out