Fellatio is the formal, almost medical name for using the mouth — lips and tongue — on a man’s penis to give sexual pleasure. English borrowed it from Latin; doctors and law books were using it in the 1800s, still half as a Latin word. Everyday English has blunter words (blow job is the common informal one). People do not say fellatio at the dinner table; they say it in textbooks, clinics, and careful writing. Strictly, the word is about a man on the receiving side. Oral sex on a woman is a different word. In loose talk some people bundle both under oral sex; this old Latin word does not. This is adult language. It is not a swear, but it is not small talk either, unless the lesson is the body or health.