The idea that opening an umbrella inside will bring you, or everyone indoors, bad luck.
This idea comes from Egypt. The furthest origin of this superstition can be found in Ancient Egypt. They didn't use umbrellas to protect them from the rain, but from the rays of the sun. It was a very valuable possession, and naturally, only nobility had the privilege. In fact, to cross into the shadow of an umbrella, even by accident, was sacrilege if you weren't of noble birth!
Ancient Egyptians also thought umbrellas could ward off evil spirits, but when you used it inside, you were insulting the sun.
Flash-forward to England, 1883. This is the first 'official' mention of the superstition. They thought holding an umbrella over your head while inside was a sign of death. In 1900, England, it was said that there would be a death in the house before the year was out, if you dared to open the doomed thing inside.