STFU is an acronym that stands for "shut the fuck up". It is often used when people are angry or upset with someone, usually online. Teenagers use this slang term to express their disagreement with another persons post or during a messaging argument. The abbreviation STFU may also be used in a joking manner. Most slang initialisms have the same meaning in upper or lower case, but for this particular slang term, using all caps rather than lowercase indicates a higher degree of anger. While the slang term STFU is usually harmless, it can come across as angry and sharp-edged, and people often use it as a harsh, immediate reply to something offensive or rude.