The black cow flies at midnight means anything you want. Also used for guy code, "is your girlfriend, wife, significant other around"? This phrase is used to check if the coast is clear.
This phrase is also used to speak in code for any real day situation: is the black cow flying at midnight?
Examples: when you don't want people around to know what you are talking about.
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.