A cake woman is a modern term for an outdated concept. This particular one refers to an unmarried woman over the age of 25 being undesirable. It's the idea that if a woman's unmarried after 25, there must be something wrong with her and comes from the idea that Christmas cake is less desirable after Christmas. Both of these ideas are wrong.
Guy 1: Damn man, she's hot.
Guy 2: Dude, you can't date her. She's a cake woman. Ya know, over 25.
Guy 1: Well, thanks for telling me her age. Imma gonna go ask her out for drinks, and you need to update your attitude.