When carrying a lock on body, which statistical data and experiences to consider re: safety concerns
To be honest, from time to time I am even afraid of the keys in my pocket, should they take some funny stance on a fall and happily pierce through my leg/waist. But that's not what you asked...

I would take a look from the other side: assuming that there will be a hit, where would it be less dangerous?

I would say that a metal object hitting your abdominal area is not going to be funny: too many things that can get damaged: spleen, liver, kidneys, ribs, pick one. So, take out holding it around your torso.

Arms and legs are less dangerous, if you accept a broken bone or a concussion.

Probably safer is a backpack: the most plausible scenario in which you will hit your back is on a flip over, and in that case I hope you wear a helmet. The backpack itself might soften the blow.

I once happened to flip over (a branch got stuck in my front wheel) and luckily my backpack went between my face and the ground. But I just had water and food in it, not a metal object.