Falar mal de alguém pelas costas.

Ela vive falando mal dos outros.
She’s always bad-mouthing other people.
Ele entrou de repente na sala e percebeu que estavam falando mal dele.
He all of a sudden walked into the room and realized they were bad-mouthing him.
O colega de trabalho falou mal dele para o chefe.
His coworker bad-mouthed him to his boss.