The definition stems back to an old art of checking out older women with kids and needing a short acronym to inform a fellow connoisseur that you have spotted a worthy trophy. This stands for Mother I'd Like to Fuck (The "to" is Silent)
acronym for "Mom I'd Like to Fuck"; generally an older woman who has beared offspring that young men fawn over. Typically she is hot, sexy, yet motherly. Popularized by the 1999 hit film American Pie and the character of "Stifler's Mom". Further popularized by the "amateur" pornographic films titled MILF Hunter, where the title character promiscuously goes around looking for MILFs to entertain for an hour or so.
"Karen recently won the title of World's Hottest MILF."