A French expression
meaning 'how are you
Literally translated it means 'how do you go?'
This is something polite that you say when you first meet someone. It is polite for this person to reply with the same question.
This expression originates from the 16/17th century. At that time people had a lot of 'digestive problems'. Many of Louis XIV's courtiers suffered with digestive problems. Their problems were caused by the huge quantities of rich food consumed in the royal court. It was not unusual for courtiers to die from over-indulgence.
Originally 'comment allez vous?' was a shortened and polite way to ask 'how do you go to the toilet?' This phrase was a polite way to inquire as to someone's health.
So remember - when you're in France make sure you ask how people are going for a shit. Maybe it's got something to do with our expression 'how do you do?'