Definition
It is too late. Particularly used of secrets that have been exposed.
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How People Actually Use It
When a woman doesn't wear any underwear, especially if her pussy is somehow exposed while not wearing underwear. You cannot call it going Commando like when a man doesn't wear underwear. It's called that because your dick is somewhat exposed to the elements.
""Dude, when the wind just blew a minute ago this hotties dress went up high and she wasn't wearing any underwear...Cat's out of the Bag!!""
Source: Urban Dictionary