The Collaborative International Dictionary of English v.0.48:

Thorn \Thorn\, v. t.
   To prick, as with a thorn. [Poetic]
   [1913 Webster]

         I am the only rose of all the stock
         That never thorn'd him.                  --Tennyson.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Thorn \Thorn\, n. [AS. [thorn]orn; akin to OS. & OFries. thorn,
   D. doorn, G. dorn, Dan. torn, Sw. t["o]rne, Icel. [thorn]orn,
   Goth. [thorn]a['u]rnus; cf. Pol. tarn, Russ. tern' the
   blackthorn, ternie thorns, Skr. t[.r][.n]a grass, blade of
   grass. [root]53.]
   1. A hard and sharp-pointed projection from a woody stem;
      usually, a branch so transformed; a spine.
      [1913 Webster]

   2. (Bot.) Any shrub or small tree which bears thorns;
      especially, any species of the genus Crataegus, as the
      hawthorn, whitethorn, cockspur thorn.
      [1913 Webster]

   3. Fig.: That which pricks or annoys as a thorn; anything
      troublesome; trouble; care.
      [1913 Webster]

            There was given to me a thorn in the flesh, the
            messenger of Satan to buffet me.      --2 Cor. xii.
                                                  7.
      [1913 Webster]

            The guilt of empire, all its thorns and cares,
            Be only mine.                         --Southern.
      [1913 Webster]

   4. The name of the Anglo-Saxon letter ?, capital form ?. It
      was used to represent both of the sounds of English th, as
      in thin, then. So called because it was the initial letter
      of thorn, a spine.
      [1913 Webster]

   Thorn apple (Bot.), Jamestown weed.

   Thorn broom (Bot.), a shrub that produces thorns.

   Thorn hedge, a hedge of thorn-bearing trees or bushes.

   Thorn devil. (Zool.) See Moloch, 2.

   Thorn hopper (Zool.), a tree hopper (Thelia crataegi)
      which lives on the thorn bush, apple tree, and allied
      trees.
      [1913 Webster]

WordNet (r) 3.0 (2006):

thorn
    n 1: something that causes irritation and annoyance; "he's a
         thorn in my flesh" [syn: irritant, thorn]
    2: a small sharp-pointed tip resembling a spike on a stem or
       leaf [syn: spine, thorn, prickle, pricker, sticker,
       spikelet]
    3: a Germanic character of runic origin

Moby Thesaurus II by Grady Ward, 1.0:

93 Moby Thesaurus words for "thorn":
   adherent, adhesive, affliction, annoyance, bane, barb, barnacle,
   bitter cup, bitter draft, bitter draught, bitter pill, bother,
   bramble, brier, bristle, bugbear, bulldog, burden, burden of care,
   burr, cactus, calamity, cankerworm of care, care, catchweed,
   cement, cleavers, cross, crown of thorns, crushing burden, curse,
   death, decal, decalcomania, destruction, disease, distress,
   encumbrance, evil, gall, gall and wormwood, glue, goose grass,
   grievance, gunk, harm, infliction, irritant, irritation, leech,
   limpet, load, molasses, mucilage, needle, nemesis, nettle,
   nuisance, open wound, oppression, pack of troubles, paste,
   peck of troubles, pest, pestilence, pine needle, plague, plaster,
   point, prickle, quill, remora, running sore, scourge,
   sea of troubles, sorrow, spicule, spiculum, spike, spikelet, spine,
   sticker, syrup, thistle, torment, torture, trouble, vexation,
   visitation, waters of bitterness, weight, woe, yucca



Easton's 1897 Bible Dictionary:

Thorn
   (1.) Heb. hedek (Prov. 15:19), rendered "brier" in Micah 7:4.
   Some thorny plant, of the Solanum family, suitable for hedges.
   This is probably the so-called "apple of Sodom," which grows
   very abundantly in the Jordan valley. "It is a shrubby plant,
   from 3 to 5 feet high, with very branching stems, thickly clad
   with spines, like those of the English brier, with leaves very
   large and woolly on the under side, and thorny on the midriff."

     (2.) Heb. kotz (Gen. 3:18; Hos. 10:8), rendered _akantha_ by
   the LXX. In the New Testament this word _akantha_ is also
   rendered "thorns" (Matt. 7:16; 13:7; Heb. 6:8). The word seems
   to denote any thorny or prickly plant (Jer. 12:13). It has been
   identified with the Ononis spinosa by some.

     (3.) Heb. na'atzutz (Isa. 7:19; 55:13). This word has been
   interpreted as denoting the Zizyphus spina Christi, or the
   jujube-tree. It is supposed by some that the crown of thorns
   placed in wanton cruelty by the Roman soldiers on our Saviour's
   brow before his crucifixion was plaited of branches of this
   tree. It overruns a great part of the Jordan valley. It is
   sometimes called the lotus-tree. "The thorns are long and sharp
   and recurved, and often create a festering wound." It often
   grows to a great size. (See CROWN OF THORNS.)

     (4.) Heb. atad (Ps. 58:9) is rendered in the LXX. and Vulgate
   by Rhamnus, or Lycium Europoeum, a thorny shrub, which is common
   all over Palestine. From its resemblance to the box it is
   frequently called the box-thorn.