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."
by kobyman November 21, 2005