The Collaborative International Dictionary of English v.0.48:

Hoary \Hoar"y\, a.
   1. White or whitish. "The hoary willows." --Addison.
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   2. White or gray with age; hoar; as, hoary hairs.
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            Reverence the hoary head.             --Dr. T.
                                                  Dwight.
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   3. Hence, remote in time past; as, hoary antiquity.
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   4. Moldy; mossy; musty. [Obs.] --Knolles.
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   5. (Zool.) Of a pale silvery gray.
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   6. (Bot.) Covered with short, dense, grayish white hairs;
      canescent.
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   Hoary bat (Zool.), an American bat (Atalapha cinerea),
      having the hair yellowish, or brown, tipped with white.
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WordNet (r) 3.0 (2006):

hoary
    adj 1: showing characteristics of age, especially having grey or
           white hair; "whose beard with age is hoar"-Coleridge;
           "nodded his hoary head" [syn: grey, gray, grey-
           haired, gray-haired, grey-headed, gray-headed,
           grizzly, hoar, hoary, white-haired]
    2: ancient; "hoary jokes" [syn: hoary, rusty]
    3: covered with fine whitish hairs or down [syn: canescent,
       hoary]

Moby Thesaurus II by Grady Ward, 1.0:

109 Moby Thesaurus words for "hoary":
   acknowledged, admitted, advanced, advanced in life,
   advanced in years, age-old, aged, ageless, along in years, ancient,
   antediluvian, antique, argent, argentine, auld, canescent, chalky,
   conventional, cretaceous, customary, dateless, elderly,
   established, fixed, fleecy-white, folk, frost-beaded,
   frost-chequered, frost-covered, frost-fettered, frost-rent,
   frostbound, frosted, frostlike, frosty, frosty-faced, gray,
   gray with age, gray-haired, gray-headed, grizzled, grizzly,
   grown old, hallowed, handed down, heroic, hoar, hoar-frosted,
   hoaryheaded, immemorial, inveterate, lactescent, legendary,
   lily-white, long-established, long-standing, marble, marmoreal,
   milky, mythological, niveous, of long standing, of old,
   of the folk, of yore, old, old as Methuselah, old as history,
   old as time, old-time, olden, oral, patriarchal, platinum,
   prescriptive, pure white, received, recognized, rime-frosted,
   rimed, rooted, senectuous, silver, silver-bearded, silver-headed,
   silvered, silvery, snow-white, snowy, swan-white, time-honored,
   timeless, timeworn, traditional, tried and true, true-blue,
   understood, unwritten, venerable, white, white as snow,
   white with age, white-bearded, white-crowned, white-haired,
   worshipful, wrinkled, wrinkly, years old