calm down, chill out.
This expression comes from the fact that Wild and Mild are opposites, yet they rhyme, and are only one letter apart, but it is the same letter, only flipped upside down.
If someone is being too wild, you can tell them to "flip their W" which is the same as telling them to be Mild instead.
"Whoa, the cops are coming! Yo, everyone's gotta flip their W's"
"Jim, you're so loud, flip your W."