A little historical insight about origins of the term 'Maverick' from this NYTimes article.
A few quotes from the article...
“I’m just enraged that McCain calls himself a maverick,” said Terrellita Maverick, 82, a San Antonio native who proudly carries the name of a family that has been known for its progressive politics since the 1600s, when an early ancestor in Boston got into trouble with the law over his agitation for the rights of indentured servants."
Considering the family’s long history of association with liberalism and progressive ideals, it should come as no surprise that Ms. Maverick insists that John McCain, who has voted so often with his party, “is in no way a maverick, in uppercase or lowercase.”
“It’s just incredible — the nerve! — to suggest that he’s not part of that Republican herd. Every time we hear it, all my children and I and all my family shrink a little and say, ‘Oh, my God, he said it again.’ ”
“He’s a Republican,” she said. “He’s branded.”