I would limit this skill to a high level thief skill, perhaps on the thuggery path. Furthermore, I would make it automatic and also make it so you had a chance of cutting your foot if it were a blade and you missed the hilt or stubbing your toe if it was a blunt weapon like a mace or staff with both results being a small -dex penalty.

This is a great idea but to me it seems like something that is tailor made for a thug.