a. A modified branch in the form of a sharp woody structure.
b. Any of various other sharp protuberances, such as a spine.
c. Any of various shrubs, trees, or woody plants bearing such sharp structures.
2. Any of various sharp protuberances on an animal.
3. One that causes sharp pain, irritation, or discomfort: He is a thorn in my side.
4. The runic letter þ, used in Old English, Middle English, and Old Norse manuscripts to represent both the voiceless sound (th) of Modern English thin and the voiced sound (th) of Modern English this, and in modern Icelandic orthography to represent the voiceless sound (th).
[Middle English, from Old English.]
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2019 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.