Definition from Wiktionary, the free dictionary
Not certain. An earlier form without the number, gallon hat, comes by folk etymology from the Spanish galón ('braid'). It may also be a hyperbolic reference to the size of the hat, or a combination of the two.
- A large cowboy hat, not holding ten gallons like the name would suggest but usually around three quarts.
- "Ten-gallon hat" in Michael Quinion, Ballyhoo, Buckaroo, and Spuds, 2004.