Hmm. I'd seen an anon claim the other day this was from a car accident and Mattis never admitted it. Now I can't find anything on a car accident at all, so maybe the only info we have really is the following article, which is just suspicion inducing. In which case, I'm a little bummed out in all honesty; I'd listened to a lot of Mattis' interviews and I quite liked the guy…
https://www.washingtonexaminer.com/policy/defense-national-security/jim-mattis-mum-on-source-of-mystery-scar#!
Reporters who met with Defense Secretary Jim Mattis Friday at the Pentagon couldn’t help but notice he had a small scar over his left eye.
When a reporter asked if he had hit his head, Mattis at first began to joke that he gotten in a bad bar fight, but then just chuckled and declined to answer.
The scar and what appeared to be some trauma to his left eye did not appear serious, but even when pressed, the Pentagon chief declined to reveal whether it was the result of an accident, injury, or medical procedure.
Afterward, his aides insisted the retired four-star Marine general, who will be 68 in September, is feeling fine and not suffering from any major health issues.