MILF is rude slang for a mother who some people find sexually attractive. It began as a joke among American college students in the early 1990s, from the crude phrase “mother I’d like to fuck” (also mom or mum in the same joke), and a 1999 film and later the internet spread it. You will see it in capital letters, like an acronym, or in small letters. It is not a polite word, not a medical word, and not a word for the classroom: it is sexual, often used by men talking about women, and it treats a person as a body. Though actually, some people now stretch it to any attractive older woman, even when she has no children — which is sloppy, and still rude. You would not say it to someone’s face unless you meant to be crude. The plain idea, without the slang, is simply an attractive mother.