thorn in one's side (or flesh)

noun

1. 1. a very short, hard, leafless branch or stem with a sharp point
   2. any small tree or shrub bearing thorns, as a hawthorn
   3. the wood of any of these trees
   4. prickle
2. a sharp, pointed protuberance on an animal; spine
3. anything that keeps troubling, vexing, or irritating one, like a constantly pricking thorn
   usually in the phrase thorn in one's side (or flesh)
4. a rune in the Old English and Old Norse alphabets (þ), used to represent the voiced or voiceless apicodental fricative: in Middle English orthography it was gradually replaced by th