thorn

Thorn comes from Old English þorn, from Proto-West Germanic *þorn, from Proto-Germanic *þurnaz, ultimately from Proto-Indo-European *tr̥nós, based on the root *(s)ter-, meaning "stiff."

The word has stayed remarkably stable in form and meaning from Old English straight through to today, always naming the sharp, rigid point on a plant.

Its relatives spread across the Germanic and Slavic worlds — Dutch doorn, German Dorn, Danish and Swedish torn — all pointing back to the same root. Curiously, the Sanskrit cognate tṛ́ṇa means "grass" rather than "thorn," showing how the same ancient idea of stiffness could branch toward different plant parts.