In medieval times, a gentleman was a man above the rank of yeoman (the owner of a small farm), but the term is now applied to a person of good manners and breeding or as a mark of respect to any man: "A gentleman should never give offense to others." "Will every gentleman please come this way?" Gentleman is now rarely used in everyday speech, but gentlemen is customary in such expressions as "What will the gentlemen have to drink?" and "Ladies and gentlemen." Lady, a term originally applied to a loaf-kneader, has been used as a polite form of address. Lady normally has no more justification than does gentleman in place of man. True, you would say to a waiter in a restaurant "The lady will have" rather than "The woman will have," but the social distinction between lady and woman has all but disappeared. Woman is a general term for the adult female human being as distinguished from the male: "This woman just gave birth to twins."
Dictionary of problem words and expressions. Harry Shaw. 1975.