A maverick is someone who thinks independently, going against orthodoxy and routinely dissenting with popular views. Many people use the term specifically to refer to politicians, although in point of fact anyone can be referred to with this term. The origins of the word are actually quite fascinating, as indeed are the roots of many words which come from the American West.
In the mid-1800s, a gentleman by the name of Samuel Augustus Maverick settled in Texas with the goal of becoming a cattle rancher. By tradition, ranchers branded their cattle with various markings to make them easy to identify, but he refused to do so. Some people have suggested that he was simply lazy, but the real explanation is probably commercial in nature. Maverick knew that, by convention, unbranded cattle belonged to whoever managed to claim them first. Using his political clout, he ensured that any unbranded cattle were regarded as his, since he was the only rancher not to use brands, and in 1867, ranchers started calling unbranded cattle “mavericks.”
It was a brilliant move. Fencing was not widely used in the American West during this period, which meant that cattle roamed freely, often having calves far from the homestead. By claiming all unbranded cattle as his own, this rancher undoubtedly acquired a few cows that actually belonged to other people. Oddly enough, it was only through political power that he accomplished this, and many modern-day mavericks claim to go against the grain of politics.
As the slang term spread across the West, people also started using it in reference to freethinking individuals, meaning that such people were not “branded” with orthodoxy. The first written record of the word in this sense appeared in a San Francisco newspaper in 1886, in a discussion of a political independent.
Individuals may still hear people refer to unbranded cattle as mavericks in some parts of the West, but most people are more familiar with this term in the sense of a rebel. In the 20th century, self-identifying with this term became quite popular, thanks to inspirations like James Dean, and some people actually worked to acquire such a reputation. Politicians in particular like to use the word in their rhetoric to suggest that the voters will be their first concern, and that they will not be afraid to defy party line if they think that the party's ideas run contrary to the interests of the people.