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.
Three nights ago, you had a remarkably vivid dream. In which you were forced, by circumstance created of your own vivid imagination, to tangle with a demon. Again, the nature of the demon or battle are yours to create.
You pitched a mighty fight, heroic even. When you woke, you had this scar.
Never waver, stick to your story! Good Luck!
Learning to juggle with swords.
Shake your head and say, “I guess I should have started with balls.”
Alternatively: Learning to juggle with balls.
Shake your head and say, “It was OK until he woke up.”
I have a scar on my right hand that sounds very similar to what you describe. People have occasionally asked about it and I (truthfully) tell them that I got it playing football in college. As a defensive tackle, I swing both arms up for a block and just happened to catch the guy’s helmet in the right way to tear a pretty big hole in the back of my hand. Not heroic, but a believable and “manly” story.
Now, the big scar on my other arm which I got when I accidentally stabbed myself with a kitchen knife is another story….
Timothy Dalton forced you to take a test of manhood on the planet of Arborea?
“You should see the other guy. They came our better than they were when i started it.”
LOL! Loving these! Happy to say the pathology report came back “benign.” I was 98% sure that it would, but an incredible amount of tension leaked out of my body when I read it, just the same.
“There I was, meditating and just about to achieve the perfect state of zenniness, when this meteorite came crashing through the tree canopy and smashed into my hand….”