A pretentious and nerdy way of describing a girl who will not succumb to the woos of potential mates (i.e., no one can have sex
with her). This term references "Fermat's Last Theorum," a mathematical conjecture that remained unproven for centuries. Only math geeks use this term.
"Suzie, Laura, and even Stacey eventually slept with some of us, but Tina remained Fermat's Last Theorum."