It stands for "Mom I'd Like to Fuck." I think it started with a website wherein Our Hero drives around supposedly picking up hot women with children and boinking them on camera. Of course, it's all real--not one of them, I'm sure, is a paid actress. A Google search for "milf" yields 5,310,000 hits.... Anyway, the acronym seems to have entered the common parlance. I have yet to see it in the OED, but I remain hopeful.