An expletive is a swear word, a curse you let out when you are startled or mad. You probably already know a lot of expletives, but you don’t need to see them here, no way in heck.
An expletive is a vulgar word that will greatly upset your grandmother if you say it in her presence. An expletive usually sneaks out because you get surprised or angry, like if you stub your toe, you might yell out an expletive. Even though you did it by accident, one of your parents still might put a bar of soap in your mouth, so watch your words. In a pinch, try these substitutes: fudge, sugar, heck, and walrus.