To be used as an alternative to the word 'excellent' in a manner whereby just enough doubt is cast in the mind of the listener as to whether you really said 'excrement' or 'excellent'. Most effective when teamed with the finger pyramid of evil contemplation and a suitably evil voice.
Person A: This cake is truly fab
Person B: Yes, it is excrement.
Person A: You won the lottery!
Person B: Excrement.