Hockey fight. All scars can be explained as the result of a hockey fight.
Human reassignment surgery? 
Car accident when I was a kid. I was too young to remember anything.
Did you ever see the movie “Jaws”?
If you’re so intent on lying about it, just say the reason for it but say you had a punctured lung from a car accident. Hit by a drunk driver, whatever, doesn’t matter.
How about it’s where your third arm used to be?
I got in Desert Storm* when I had to eject from my F-18 after being hit by a surface to air missile.
*adjust conflict to agree with your age.
Well as a person who has a large scar that comes out whenever I wear shorts. I get questions about it mainly because I forget who hasn’t seen it so i forget to tell them about it. So I don’t mind questions. But I have the opposite problem mine is a football injury and as such is boring but i have invented stories that are actually interesting and over the years have invented quite a back-story based on the questions I been asked.
So if you don’t want people to know the truth I’d go that route. My friend’s dad is a cop and he told us about a call out he had once where the guy was impaled on a stop sign but still alive I’m sure that would leave a similar scar and then you can talk about a wild car ride it can get fun.
Personally I tell people that a shark bit my leg off just below the knee but spit it out so I beat him with it luckily the doctors were able to reattach it. I’ve got more detail if ya want it.
Shake your head slowly “When a Vegas showman tries to sell you a used tiger, just say no.”
When asked a similar question, I replied, “If it were any of your business you would know already”. Worked for me.
I love this one!
“That’s where Lord Voldemort’s curse rebounded on him and destroyed his physical form.”
Sailboat
I live in Los Angeles, and I like to go to the beach. I suppose I could wear a shirt to go into the water, but that seems weird.
Our company picnic was at the beach as well. I didn’t go into the water because I didn’t feel like taking my shirt off and getting peppered with questions. But then people kept asking me why I wasn’t in the water.
Well, matey, methinks it would be best if ye just tells them that you used to be a Pirate.
Har.
Just say what I always say: “I’m not paying you to talk!”
“I was gored by an H’Elephant whilst on safari in H’Africa, doncha knaw, old boy.”
Slate’s original Dear Prudie mentioned a woman who, when asked an invasive question by a total stranger, gave a cold direct stare and asked “Do I know you?”
I’m generally not the types to feel affronted enough to employ that response, but I do keep it at the ready
Ooooh, I like that one, gotta remember it!
Only if you say your call sign was “Maverick” 