A simple attempt to measure and quantify the amount of alcohol needed in order for you (or your friend) to willingly copulate with any given female. A fun game to play in a pub, the standard of measurement is one draw (draft) of an average strength American beer. While a reasonably attractive female may rate a 5-6 beer factor, a more homely specimen may require a 12 or 15 beer factor. On the other hand, a very attractive female may not warrant the need for any beer at all, resulting in the prized "ZBF" or "Zero Beer Factor".
Frank was scoping over some fat broad playing pool in the back room. She looked like a dyke to me. She was so fugly that she was probably a 24 beer factor.