speak badly of him
陰で彼の悪口を言うな。
Don't talk badly about him behind his back.
彼女はよく彼の悪口を言う。
She often badmouths him.
彼の悪口を言わないで。
Don't badmouth him.
そんなに彼の悪口を言うべきではありません。
You shouldn't speak so badly of him.
人前で彼の悪口を言うな。
Don't badmouth him in public.
彼の悪口をいっても何もならない。
Badmouthing him won't accomplish anything.