Ninjas. Always ninjas.
Fill in whatever story you want for why you had to fight ninjas. Better, you can alter the story with each telling to people who have not heard it before.
Dueling scar from an evil finger puppet.
Unca Pete was missing fingers. The two in the middle, left hand(dubious sawmill accident 
).
If you asked him what happened he had many come backs. My favorite was “I scratched my ass and my tape worm bit me!”
Reminds me of an old Rockford Files show. I thing maybe did really have a broken arm, he was in a cast.
People kept asking him how he broke it “I fell off my skate board” is a favorite of mine.
Jimbo on a skate board, snerk.