STFU is an acronym that stands for "shut the fuck up". It is a vulgar expression that is often used when people are angry or upset with someone, usually online. The phrase is most commonly used to demand someone to stop speaking or writing because the speaker finds the other persons words annoying, inappropriate, or offensive. It is often used as a form of aggressive expression in response to another persons statement or behavior. The phrase "STFU" is typically used in spoken language, often to interrupt or dismiss someone speaking. It is intended to show that the speaker wants the other person to stop talking.