The bell-bottom jean was extremely popular during the 1960s and 70s. It had a tight waist and thighs, but flared below the knees. This gave the jean a vague resemblance to the shape of a bell, which is supposedly the origin of the name.
Allegedly, this kind of jean was first spotted with US navy men wearing the bell-shaped pants, although clothing could vary per ship.
Source: Nouvelle de Nîmes Nº 5: The Denim Dictionary