A libertine is someone who lives for pleasure, especially sexual pleasure, and does not care much about the moral rules other people follow. It is mostly a word for the person; you can also hear it as a describing word (a libertine life). The tone is old-fashioned and a little literary — novels, history, and sharp comments use it more than everyday chat. You would not usually call a classmate this; it is a strong, judging word. Though actually the story under it is not about sex at first. In Latin, a libertinus was a freedman: a person who had been a slave and was then made free. From that came the idea of someone who had slipped the rules. French libertin carried it into English. So the “free” inside the word is the same family as liberty, but the meaning drifted from “set free” to “free of moral rules, often in bed.” It is not the same as libertarian, which is about government and politics.