Bell-bottoms (or flares) are a style of trousers that become wider from the knees downward, forming a bell-like shape of the trouser leg. In the early 19th century, when a standardized uniform did not yet exist in the U.S. Navy, some sailors adopted a style of wide trousers ending in bell-shaped cuffs. In 1813, one of the first recorded descriptions of sailors' uniforms, written by Commodore Stephen Decatur, noted that the men on the frigates United States and Macedonia were wearing "glazed canvas hats with stiff brims, decked with streamers of ribbon, blue jackets buttoned loosely over waistcoats, and blue trousers with bell bottoms."

Source: Wikipedia