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) alternative meaning of “common” – the logic being that because things that are common (or shared – “gemeinsam”) can’t be of high value, the word developed a negative connotation in both languages:

– “Du bist so gemein zu mir!” – “You’re (being) so mean to me!”
– “Hör auf so gemein zu sein!” – “Stop being so mean!”
– “Das gemeine Volk versteht das nicht.” – “The common people won’t understand that.”

Alternatively, the word “böse” (which can also mean “evil”, “angry” and “naughty”:

– “Es ist ein böses Kind.” – “It’s a mean/angry/naughty/evil child.”
– “Er ist böse auf mich.” – “He’s mad at me.”

More colloquially, “fies” or “mies” can also be used to express that someone is mean. Relatedly, “hinterhältig” means “devious” or “sneaky”.