How do you say that someone’s mean in German?

The English word “mean”, in the sense of “wicked” or “vicious”, is gemein in German. Both “mean” and gemein actually share the origin and a (somewhat antiquated) meaning of “common” - the logic being that things that are common or shared (gemeinsam) can’t be of high value, so the word developed a negative connotation in both languages.

Alternatively, you can use the word böse (which can also mean “evil”, “angry” and “naughty”), or the more colloquial fies or mies. Mies can mean “mean” or “nasty” about people or behavior, and “lousy” about quality: Das Wetter ist mies. Relatedly, hinterhältig means “devious” or “sneaky”.

Examples