A killjoy is a person who spoils other people’s fun. When everyone else is laughing, dancing, or enjoying a party, the killjoy stops the pleasure — by complaining, by being very strict, or by bringing in something gloomy. People use it as a slightly unkind name, often while teasing: “Don’t be such a killjoy.” It is not a swear word, but it is not a kind word either. You might call someone a killjoy for a small thing, like turning the music off, or for a bigger one, like shutting down a whole celebration. Though actually, the person getting that name may only be trying to be careful, fair, or quiet; the word shows how the others feel in that moment, not the whole truth about that person. It is a noun for a kind of person. Everyday English does not really use killjoy as a verb.