Un navet has a double meaning. First it refers to a root vegetable, the turnip. But in everyday French, un navet is used to describe a bad movie, a flop. But why do French people call a bad movie un navet? Well, the turnip has never been very popular and has been left out of modern recipes.
The expression was already used in the 13th century to describe something of little value or not deserving much respect. With the advent of cinema, it became familiar to describe terrible films.