From Longman Dictionary of Contemporary Englishfour-letter wordˌfour-letter ˈword noun [countable] SWEARa word that is considered very rude and offensive, especially one relating to sex or body waste SYN swearword
Examples from the Corpus
four-letter word• "Diet" has become a four-letter word in the commercial weight-loss business; now they're called "foodplans."• The best four-letter word in the world.• No four-letter words, please - this is a family show. 