A person who criticizes someone secretly or speaks maliciously about someone who is not present.

पीठ में छुरा घोंकने वाला

English Usage: She always accused him of being a backbiter who spread rumors about her among friends.

Hindi Usage: उसने हमेशा उसे पीठ में छुरा घोंकने वाला कहा जो उसके बारे में दोस्तों में अफवाहें फैलाता था।