Bell-bottoms are trousers that turn into wider from the knees downwards. Related styles comprise flare, loon slacks and boot-cut/leg trousers. Hip-huggers are bell-bottomed, flare, or boot-cut pants that are built-in tightly around the hips and thighs worn by men and women.

Bell-bottoms precise origins are uncertain. In the early nineteenth century, very wide pants ending in a bell began to be worn in the U.S. Navy (Clothing varied between ships, however, since in the early days of the U.S. Navy each ship's captain decided what clothing to buy for his men). In one of the first recorded descriptions of sailors' uniforms, Commodore Stephen Decatur wrote in 1813 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 ters with bell bottoms." Though the British Royal Navy usually was the leader in nautical fashion, bell-bottoms did not become regulation wear for the Royal Navy until the mid-1800s.

These "bell-bottoms" were often just very wide-legged trousers, unlike modern versions cut with a distinct bell. While many reasons to explain sailors' wearing of this style have been cited over the years, most theories have little credibility because reliable documentation is lacking. In the 1960s, at least, U.S. naval recruits were taught to use their bellbottoms as life preservers by slipping them off, then tying the legs shut and capturing air in them.