A hairdo is the style someone’s hair is cut or shaped into — a bun, waves, a big 1960s pile, a neat office look. The word is friendly and a little old-fashioned. A grandmother might say “What a nice hairdo!”; a teenager is more likely to say hairstyle, or just hair, or the name of the style. Though actually that “old” feel is part of how the word lives: it can sound warm, or slightly comic, if you use it for a very serious cut. It comes from the everyday phrase do your hair — get it ready, put it up, make it presentable. The result of that “doing” is a do, and hairdo is the full name. It is countable: a hairdo, her new hairdo. It is about the look, not about washing. A haircut is specifically the cutting; a hairdo can be cutting, pinning, curling, or all of it together, as long as you can see a finished style.