A shillelagh is a stick that is sometimes used as a club, like a bat or a hammer. It's made from the wood of a special kind of tree that grows in Ireland called a blackthorn. The stick is usually about two or three feet long and has a thick end that is used for hitting things. The word "shillelagh" comes from an Irish word that means "little oak forest", which is pretty cool.
People have been using shillelaghs for a long time, and they're often associated with Ireland and Irish culture. Lots of people like shillelaghs because they're strong and easy to use. They're also sometimes decorated with carvings or symbols that represent different things.
But remember, hitting people or animals with a shillelagh is not nice and can hurt them really badly, so it's important to always use it for fun or as a decoration!