Why is walking under ladders believed to be unlucky?

There are a few explanations for this. The most common is that someone on the ladder might be holding a pot of paint or a bucket of water which could drop on you if you walked underneath.

Another explanation relates to the tyburn gallows at what is now Marble Arch in London. Until 1783 criminals climbed a ladder to the gallows with a rope round their neck and this was then kicked from under them. The belief grew that walking under a ladder invited death.