Dang it! You done figured it out already, Sambo? An' here I thought we were bein' all clever and stealth-like. :)
A ten-gallon hat is your stereotypical American oowboy hat. Think "The Lone Ranger" and all that. For a little more background:
The Stetson company and Texas Bix Bender (who wrote Hats and the Cowboys Who Wear Them) say that "ten-gallon" refers to how much liquid (certainly not brains) such hats could carry in their crowns. This story is wrong. John Batterson Stetson (1830-1906), who is sometimes credited with inventing the cowboy hat, was from New Jersey. 'Nuff said. You'll also find the myth that ten-gallon hat referred to the unit of capacity in The Cowboy Encyclopedia by Dr. Richard W. Slatta. Slatta is from North Carolina. Uh huh.
So here is la pura verdad: "Ten-gallon hat" is the result of a linguistic mix-up. "Galón" is the Spanish word for "braid." Some vaqueros wore as many as ten braided hatbands on their sombreros, and those were called "ten galón hats." English speakers heard gallon. Real cowboy hats came to Texas from the Spanish via Mexico (unless you want to go all the way back to Genghis Khan and the Mongolian horsemen, who apparently wore something similar).
Hope that clears it up. Once again, it was some dumba$$ murdering a language and coming up with the wrong meaning in their own native tongue.
So endeth todays history lesson. :)
Regs,
Professor Jeff-Bob