the expression you use when you can't form a coherent sentence to describe how horrid a person/place/thing is.
Note: the "lion" part can be exchanged for any other animal, as long as it isn't a cute animal (bunny, puppy, pony, etc.).
"wow, look at that milf over there."
"dude, she's a total sebastian lion."
"oh look, timmy and cindy are dancing together."
"aww, it's so great when two sebastian lions hook up."
"yeah, so the rest of us don't have to worry about them asking us out. *shudder*."