For reasons that are too complicated to explain here (or, rather, I'll explain them later), most of the English as a Second Language dictionaries sold in Spain are from the UK -- Oxford, Longman and Cambridge, for example. American and Canadian students, however, typically use Merriam-Webster dictionaries, which were first published in the mid-19th century in the United States.
Merriam-Webster's website is a great place to find tons of fun information about English, and they've recently added a new feature: Ask the Editor. These videos give you short explanations (under 2 minutes) of the origins of certain words and expressions.
Here's one for you sports fans who want to know the origins of the noun "hat trick":