When you give someone an earful, you angrily tell the person what you think. Your teacher might give the class an earful when no one has finished yesterday's homework.
The informal word earful is good for describing a lengthy reprimand or angry criticism. A furious senator might give her colleagues an earful when they can't reach a compromise, and a frustrated father might give his kids an earful after losing track of them in a busy shopping mall. Earful was first used in the mid-1900's, from the Old English roots eare, or "ear," and the suffix ful, "full or full of."