What does "prune" mean here? Isn't it fruit?

Rebecca

A prune is a type of preserved plum, but is also used as slang to describe an "unpleasant or disagreeable person." Hagrid has used "you great prune" here to show his dissatisfaction with Dudley. Ex: Mary is one of the smartest students in our grade, but everyone thinks she's a miserable prune. Ex: Dudley is famous for being a big prune.