This is a very strong swear word for a person you are angry with, or who you think is stupid or has treated you badly. Do not use this word. In English-speaking countries it can start a fight; at school or at work it can get you into real trouble. People also stretch it. “You lucky fucker” can sound almost friendly, like “you lucky thing,” and “poor fucker” can even sound sorry for someone. Angry people also point it at a thing that will not work: “this fucker won’t start.” Though actually, a learner cannot hear those lines safely yet — the word is still dirty, and the softer reading only works when both people already know each other well. It can be said of a man or a woman. The old “person who has sex” meaning sits under it, which is why it feels so rude, but in real talk the insult is the one you will hear. Street talk, never polite, never for a teacher.