The newest way of stating that someone is lying. Also can be used as a "reverse psychology' to get someone to prove something.
John: The girl I went out with last night was so sexy.
James: Lion Ass!! You were out with your brother last night! I saw you!!
Carlos: I got a new tattoo!!!
Jeremy: Ooh, cool! Send a pic!
Carlos: Uhh, I cant.
Jeremy: Why not? Hmm, Lion ass!! you aint got no tattoo!!! Prove it!! Pics or lie!!