1. talking shit indirectly about a person (usually mean)
2. Talking shit in ear-shot, loud enough to hear, but to another person about some one.
1. A) did you hear about how much of a slut Kelly is?
B) dude shut up! You are talking behind her back
2. Kelly and Kelsey are standing together, Hannah is a few feet away with another friend.
K) Dude, Hannah's being a bitch. She just needs to get a life and realize no one likes her.
H) You know i can hear you?
K) I know, that's the point, Talking behind someone's back is sooo mean