Cowboy fuel is a name given to an alcoholic beverage which generally promotes cowboy like behavior.

The most renown cowboy fuel would be a Desperado, which is a tequila flavored beer.

cowboy fuel generally leads to the worst headache of your life and you feel very drained the next day, assuming you didn't get too pumped up and ended in the cells or A&E.

A casual invitation / threat used by an individual that is not afraid of fucking someone up.