Mumming is the old custom of dressing up — often with a mask or disguise — and going around to act a simple traditional play. A mummer is the actor; mumming is the doing. You meet the word in writing about British and Irish folk festivals, especially around Christmas. Dictionaries sometimes describe it as wordless acting, and the name does hint at keeping mum. Though actually the plays often have simple spoken parts and songs. It is not a word for ordinary school drama, and it has nothing to do with Egyptian mummies.