If you did it today, it wouldn't be the first. Here are some:

• The Unbreakable Umbrella is the first one I can recall hearing of. It's heavy (775g) and made of a composite material the manufacturer claims is good for hitting people.
• The Streetwise Stunbrella conceals an electroshock weapon inside an umbrella.
• Security Umbrella is another umbrella marketed as suitable for hitting people.
• "Bulgarian umbrellas" are poison pellet guns built into umbrellas, intended for murder.
• Windlass Steelcrafts sells an umbrella which conceals a rapier. There appear to be several similar products from other companies.