In English, it would go something like XYZ (examine your zipper).
But how could we say that in Korean? It's quite simple... the phrase you're looking for is...
Namdaemun is the south gate and when something is yeolyeodda, it's open.
So, South gate is open.
It's quite easy to remember too, just think south, lower part, and open.
There are many stories to how this came about. One of the story is that long time when the usually closed south gate in Seoul, Namdaemun, was open, that's when people could see things that they usually weren't privileged to and hence the lower part should be covered most of the time the once in awhile agape situation could be viewed as how it was back then. Another way to interpret it is the word 'nam' means male and 'mun' means door, so, the door to your male part is open.
So next time you see someone's zipper undone, try this phrase:
Your south gate is open!