Where I come from, a "T-back" is a thong riding so high over low-rise jeans than people behind her can see flesh underneath. Generally in NYC it's not a good thing, as most toursits have not seem to have figured out that low-risers are a privelidge, not a right.
They vote on things like that in NY? And incidentally, if you're in NY, that explains a lot...
Yep, you have T-back's nailed.