Well, the original Mr. Terrific had the words FAIR PLAY on his chest.
Seriously.
The modern version has a stylized “T” pattern on his shirt (and “FAIRPLAY” written on his sleeves).
Yep, I believe this is the current official explanation (until the decide to change it again) and that it was introduced in The Kents a twelve-issue mini-series from a few years ago that told the story of the history of thr Kent family in the years before, during and after the Civil War, mixing real history with fictional characters. The series was a great read, but the whole S symbol coming from an indian medicine blanket thing is corny as hell.
Shhhh… don’t tell anyone! 
Yeah, it’s not something I’d be proud of, either.