of all the silly superstitions in the world, this one is the best.

[levity]

My brother got his arm pooped on by a bird. Why are you supposed to be lucky if you get pooped on by a bird? Of all the things that bird poop can hit, it has to be you, and you're considered lucky?

My guess is people made this one up so that the people who did get pooped on wouldn't feel so sorry for themselves.