1.  A hard and sharp-pointed projection from a woody stem; usually, a branch so transformed; a spine. [1913 Webster]

2.  Any shrub or small tree which bears thorns; especially, any species of the genus Cratægus, 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." [1913 Webster]
"The guilt of empire, all its thorns and cares,
Be only mine." [1913 Webster]

4.  The name of the Anglo-Saxon letter th, as in thin, then. So called because it was the initial letter of thorn, a spine. [1913 Webster]

   To prick, as with a thorn. [1913 Webster]
"I am the only rose of all the stock
That never thorn'd him." [1913 Webster]

thorn, n.
1 a stiff sharp-pointed projection on a plant.
2 a thorn-bearing shrub or tree.
3 the name of an Old English and Icelandic runic letter, = th.

Idiom:
on thorns continuously uneasy esp. in fear of being detected. thorn-apple
1 a poisonous plant of the nightshade family, Datura stramonium.
2 the prickly fruit of this. a thorn in one's flesh (or side) a constant annoyance.

Derivative:
thornless adj. thornproof adj.

Etymology:
OE f. Gmc

N  bane, curse, evil, hurtfulness, painfulness, scourge, damnosa hereditas, white elephant, sting, fang, thorn, tang, bramble, brier, nettle, poison, toxin, teratogen, leaven, virus venom, arsenic, antimony, tartar emetic, strychnine, nicotine, miasma, miasm, mephitis, malaria, azote, sewer gas, pest, Albany hemp, arsenious oxide, arsenious acid, bichloride of mercury, carbonic acid, carbonic gas, choke damp, corrosive sublimate, fire damp, hydrocyanic acid, cyanide, Prussic acid, hydrogen cyanide, marsh gas, nux vomica, ratsbane, hemlock, hellebore, nightshade, belladonna, henbane, aconite, banewort, bhang, ganja, hashish, Upas tree, Toxline (online), rust, worm, helminth, moth, moth and rust, fungus, mildew, dry rot, canker, cankerworm, cancer, torpedo, viper, demon, toxicology, baneful, poisonous, phr, bibere venenum in auro, Contingent Subservience.

N  painfulness, trouble, care, trial, affliction, infliction, blow, stroke, burden, load, curse, bitter pill, bitter draught, waters of bitterness, annoyance, grievance, nuisance, vexation, mortification, sickener, bore, bother, pother, hot water, sea of troubles, hornet's nest, plague, pest, cancer, ulcer, sting, thorn, canker, scorpion, dagger, scourge, carking care, canker worm of care, mishap, misfortune, desagrement, esclandre, rub, source of irritation, source of annoyance, wound, open sore, sore subject, skeleton in the closet, thorn in the flesh, thorn in one's side, where the shoe pinches, gall and wormwood, sorry sight, heavy news, provocation, affront, head and front of one's offending, infestation, molestation, malignity, causing pain, hurting, hurtful, painful, dolorific, dolorous, unpleasant, unpleasing, displeasing, disagreeable, unpalatable, bitter, distasteful, uninviting, unwelcome, undesirable, undesired, obnoxious, unacceptable, unpopular, thankless, unsatisfactory, untoward, unlucky, uncomfortable, distressing, afflicting, afflictive, joyless, cheerless, comfortless, dismal, disheartening, depressing, depressive, dreary, melancholy, grievous, piteous, woeful, rueful, mournful, deplorable, pitiable, lamentable, sad, affecting, touching, pathetic, irritating, provoking, stinging, annoying, aggravating, mortifying, galling, unaccommodating, invidious, vexatious, troublesome, tiresome, irksome, wearisome, plaguing, plaguy, awkward, importunate, teasing, pestering, bothering, harassing, worrying, tormenting, carking, intolerable, insufferable, insupportable, unbearable, unendurable, past bearing, not to be borne, not to be endured, more than flesh and blood can bear, enough to drive one mad, enough to provoke a saint, enough to make a parson swear, enough to gag a maggot, shocking, terrific, grim, appalling, crushing, dreadful, fearful, frightful, thrilling, tremendous, dire, heart-breaking, heart-rending, heart-wounding, heart-corroding, heart-sickening, harrowing, rending, odious, hateful, execrable, repulsive, repellent, abhorrent, horrid, horrible, horrific, horrifying, offensive, nauseous, nauseating, disgusting, sickening, revolting, nasty, loathsome, loathful, fulsome, vile, hideous, sharp, acute, sore, severe, grave, hard, harsh, cruel, biting, caustic, cutting, corroding, consuming, racking, excruciating, searching, grinding, grating, agonizing, envenomed, catheretic, pyrotic, ruinous, disastrous, calamitous, tragical, desolating, withering, burdensome, onerous, oppressive, cumbrous, cumbersome, painfully, with pain, deuced, Int, hinc illae lachrymae!, surgit amari aliquid, the place being too hot to hold one, the iron entering into the soul, he jests at scars that never felt a wound, I must be cruel only to be kind, what deep wounds ever closed without a scar?.

christ thorn, cockspur thorn, common thorn apple, evergreen thorn, fire thorn, glastonbury thorn, hedge thorn, jerusalem thorn, mysore thorn, parsley-leaved thorn, thorn apple