Use prude to describe someone who is too concerned with being proper or modest. It is a derogatory label affixed most often to people who are not forthcoming romantically — and it's not very nice.
To be prude is to be prudent, or careful. That’s not a bad thing, but prude is usually an insult. A prude might gasp when someone says a bad word. Prude was borrowed from French, short for prude femme "wise and good woman." Though the word took on a negative meaning, Old French prud also meant "capable or brave" and is probably the source of English proud.