Fink is a word that refers to someone who informs on others, especially by telling authorities about someone else's wrongdoing or secrets. If a student tells the principal which classmate pulled the fire alarm, the other kids might call that student a fink. The word carries a strongly negative tone: it suggests betrayal and disloyalty rather than honest reporting.

The word became popular in American slang during the early 1900s and was especially common in labor union contexts, where a fink was someone who reported union activities to company bosses. Later, it spread to schools, neighborhoods, and anywhere people valued loyalty to their group.

People sometimes talk about a difference between being a fink and reporting something serious. For example, telling a teacher that someone brought a weapon to school is protecting people from real danger. But telling on a friend for chewing gum in class might earn you that label.

Today, people also use words like “tattletale” or “snitch” to mean roughly the same thing, though fink sounds more old-fashioned.