What do we mean when we say it today?
Well aren't you a sorry sight? If you've ever had someone say that to you, don't worry. It doesn't mean what Macbeth says. No one's saying they're regretful or distressed at seeing you. Phew.
Nowadays, Shakespeare's little phrase means that someone has an untidy appearance or is a bit messy. It's really just a nicer way of saying you don't look so hot today.