Christmas riddle.

This, dear readers, is a highly creative and rather cryptical variant of the standard "Flush after use" (which is what it says in Chinese).

Not only did the Beijing restaurant owner take a "W" and turned it upside down. He seemed to have come up with a whole new word. Native speakers, what on earth is meant by "mang out"? Looking forward to your creative postings.

Many thanks, Namisan!