thorn

Concise Oxford English Dictionary © 2008 Oxford University Press:
thorn/θɔːn/

▶noun

• 1 a stiff, sharp-pointed woody projection on the stem or other part of a plant.

• 2 a thorny bush, shrub, or tree, especially a hawthorn.

• 3 an Old English and Icelandic runic letter, þ or Þ, representing the dental fricatives /ð/ and /θ/, eventually superseded by the digraph th.

– phrases
a thorn in someone's side (or flesh) a source of continual annoyance or trouble.

– derivatives
thorned adjective,
thornless adjective.

– origin OE, of Gmc origin; the OE letter was named from the word of which it was the first letter.