A sharp pointed protective structure on plants; something that causes persistent trouble or pain.
From Old English þorn, related to Old Norse þorn and German Dorn, all from Proto-Germanic thurnu-. The metaphorical use meaning 'source of trouble' appears in early English, as in 'thorn in one's side' from biblical sources.
The letter 'thorn' (þ) was actually part of Old English and represented the 'th' sound we now write with two letters. Medieval scribes gradually replaced it with 'th,' but it survived in Iceland and gave us the 'ye olde' spelling - the 'y' was actually a thorn that looked similar to 'y' in some fonts.
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