I can't think of many/any places where this kind of concealed weapon would be legal, but I can think of one place we can admire its craftsmanship: here.
YouTube user ammnra is the man behind it, wisely keeping the footage restricted to him testing the blade in his bathroom, and not running around on rooftops looking for men in silly hats carrying crossbows.
This isn't the first time we've featured his work: last year, he made one of the best cosplay outfits we've ever seen, turning a man into alien vigilante Garrus from the Mass Effect universe.
[via Reddit]