Even if it was SDT, there really really is for sure a Sigma Tau Delta
I got a letter of eligibility from them once (haven't joined yet tho) and the girl that wrote it had a good sense of humor about thier name. She was like... now for the first STD joke.. it's the first one you'd want to write home about! eesh... I'm not sure why those letters or how long they've been around, but maybe before the acronym for sexually transmitted diseases became well-known I would guess!
And I have also never seen somebody actually wearing their STD letters on campus... heh