Unconfirmed. Hypotheses include:
- Middle English greene horn, the horn of a newly slaughtered animal
- From the reference to an animal with green (that is, young) horns
- From 17th century jewelery manufacturing which used decorated horn which was impressed at a specific temperature. If heated too high (a common mistake by apprentices), the horn turned green.
greenhorn (plural greenhorns)
- See also Wikisaurus:beginner