The popular image of a cowboy would not be complete without the wide-brimmed "10-gallon hat," yet even the most hardened cattlemen can't agree on how the iconic headgear first got its name. The conventional explanation is that "10-gallon" refers to how much liquid could be carried inside the hat. Most experts argue that the name "10-gallon hat" is actually an import from south of the border. Cattle drivers and ranchers in Texas and the Southwest often crossed paths with Mexican vaqueros who sported braided hatbands—called "galóns" in Spanish—on their sombreros. A "10 galón" sombrero was a hat with a tall enough crown that it could hold 10 hatbands, but American cowboys may have anglicized the word to "gallon" and started referring to their own sombrero-inspired headgear as "10-gallon hats."

Source: The Hatterist