Stands for my young one. It can be used only on people younger than you, even if its only by 4 days. It is a secret term of endearment, for when you would rather use a real term of endearment like babe but don't want to be obvious.
It makes girls smile when you say it to them, especially when you text them, "Goodnight.... myo"
It does sound slightly condescending in a way but it definitely isn't.
Girl: NO! I'm not yours!
Guy: YES! You are!
Girl: okay fine...