Mean-spirited or malicious — behaving unkindly to someone, such as teasing or bothering them; also describes a person with such a disposition.
友達に意地悪をしてはいけないと先生に注意された。
The teacher warned us not to be mean to our friends.
彼女は冗談のつもりで言ったが、それが意地悪に聞こえた。
She meant it as a joke, but it sounded mean.
兄は弟に意地悪をして、弟はよく泣いていた。
The older brother was mean to his younger brother, who often cried.
あの人の意地悪な態度に皆が困っている。
Everyone is troubled by that person's nasty behavior.