Definition from Wiktionary, the free dictionary
prude (plural prudes)
- A person who is or tries to be excessively proper, especially one who is easily offended by matters of a sexual nature.
1991, Robert M. Pirsig, Lila:
- If you didn't go for Lila you're some kind of prissy old prude. If you did go for her you were some kind of dirty old man.
A person who is or tries to be excessively proper
- prude in Webster’s Revised Unabridged Dictionary, G. & C. Merriam, 1913
- prude in The Century Dictionary, The Century Co., New York, 1911
In widespread use but still not even mentioned in major dictionaries and therefore widely considered "ungrammatical" and nonstandard for "be a prude".