In British superstition, opening an umbrella indoors is traditionally thought to bring bad luck.
The belief is often explained through practical history. Early umbrellas could be stiff, awkward objects with sharp ribs and difficult mechanisms. Opening one in a crowded room could injure someone, knock over objects, or damage furniture. A warning against unsafe indoor use may therefore have helped develop into a broader supernatural rule.
The superstition is not limited to Britain, although it is strongly associated with English-speaking popular culture. Some explanations instead connect umbrellas with protection from the sky and argue that using one under a roof symbolically challenges household spirits. These interpretations are folklore theories rather than proven origins.
Modern umbrellas are safer and more compact, but opening one indoors can still be inconvenient and may damage nearby objects. The belief is consequently often presented as a playful taboo with a practical behavior behind it, rather than as a claim supported by evidence.