casual sex noun[ U ] uk /ˌkæʒ.ju.əl ˈseks/ us /ˌkæʒ.uː.əl ˈseks/ sexual activity between two people who are not in a relationship: They were having casual sex but it never became a serious relationship. Young people need to be aware of the risks involved in casual sex. See
casual
sex We both knew that it was casual sex and there were no feelings involved. He often went to clubs looking to pick up partners for casual sex. Concerned parents are looking for ways to discourage teenagers from having casual sex.