When one says something, but usually means the opposite.
Used sometimes in order to belittle someone and make them seem retarded... or equally to amuse and impress them with your mind. Indeed.
Betty: June hi! how are you today?
June: Menstral cycle - it's like I got a fire and ice show going off in my lower regions. It's fantastic.
Betty: Owh, doesn't sound it!
June: Umm... sarcasm?
Betty: No I'm not being sarcastic, it really doesn't sound it!
June: *slaps head*