michaelr took the words right out of my nose, er, I mean mouth.

Actually, a sudden and unexpected guffaw could cause liquid to escape either orifice, depending upon the stage of consumption at which the laugher is caught.

I do sometimes use this expression figuratively rather than literally, but I have certainly experienced actual involuntary spewing of a beverage more than once during my 60 years on this earth.

I will also sometmes report that "I almost peed on myself." When I say that, I am exaggerating. I have yet to actually urinate involuntarily just because something's funny.

Of course, there's a first time for everything, and I'm not getting any younger...