"F" funding is an entrepreneurial slang term for funding that comes from ...
... Founders, Family, Friends, Fanatics, and Fools. That is, venture funding that comes directly from the founders (maxing out their credit cards?), the founders' family members (maxing out their credit cards, too?), the founders' friends, individuals that are fanatic about the venture, and fools (a semi-affectionate term for individuals that invest in a venture with a minimal level of due diligence, investigation, or expectation of a return on their investment). "F" funding is the most common source of initial capital for most independent start-up ventures, and often the only source.