Yes, it is can be. Or, well, was. As said, they were designed to be weapons.
They could be crazy leathal if put in the right hands. Have you ever seen some of those pro yo-yo users do tricks? Imagine stuff like that...but going straight for your face.
I always imagined that they could be sharpened on the edges of the wheels also. Which would be smashing.
well if the yo-yo had SPINES on it metal spines mind you, It would be as lethal as a mace since a small heavy object could most likely hit harder than aa large object at a slow speed. and I bet you could point to one thing on your desk and say "omgosh! I could kill somewone with that!" (I am NOT repeat, NOT giving any ideas for the very deppressed or bullied [this is NOT a joke, seriously im really serious about this])
Well, it seems that there is a lot on different answers as far as that goes. Some people say "No, it was never a weapon." while others say "It was a weapon hundreds of years ago."
So it is up to the individual to decide I suppose. Not that it really matters. I could still probably kill you with a properly made yo-yo regardless of what it was made for...
I can't find evidence supporting the theory. Looks like they were first invented in China and given as toys. To me, though, the question is still a valid one: a yo-yo can be used as a weapon, even if they weren't invented as weapons.