We found one dictionary that defines the word thorn in someone's side:

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1. thorn in someone's side: Wiktionary

Definitions from Wiktionary (thorn in someone's side) ▸ noun:  (idiomatic) A persistent annoyance. ▸ Words similar to thorn in someone's side ▸ Usage examples for thorn in someone's side ▸ Idioms related to thorn in someone's side ▸ Wikipedia articles (New!) ▸ Words that often appear near thorn in someone's side ▸ Rhymes of thorn in someone's side ▸ Invented words related to thorn in someone's side