Old English þorn "sharp point on a stem or branch," earlier "thorny tree or plant," from Proto-Germanic *thurnuz (source also of Old Saxon, Old Frisian thorn, Dutch doorn, Old High German dorn, German Dorn, Old Norse þorn, Gothic þaurnus), from PIE *trnus (source also of Old Church Slavonic trunu "thorn," Sanskrit trnam "blade of grass," Greek ternax "stalk of the cactus," Irish trainin "blade of grass"), from *(s)ter-n- "thorny plant," perhaps from root *ster- (1) "stiff."
Figurative sense of "anything which causes pain" is recorded from early 13c. (thorn in the flesh is from II Corinthians xii.7). Also an Anglo-Saxon and Icelandic runic letter (þ), named for the word of which it was the initial (see th).
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