It's from Adventure Time:

> 
> We'll work together and use that serum you made. The one that was "Ah!
>  So Spice!" And spice-bomb him non-stop **'till his butt falls out**! How's
>  that sound?
> 
> 
> 

To make it clear, they're just pranking the guy to irritate him and make him go away.

What does this phrase mean, both literally and idiomatically?

I know that hair or teeth can fall out, but I can't picture a butt fall out.

Or does *fall out* mean *leave* here as in:

> 
> It's late, George. I have to fall out.
> 
> 
>