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.