Confederate (noun)

1. A person one works with, especially in something secret or illegal; an accomplice.
2. A supporter of the Confederate States of America during the American Civil War.

Origin:

From latin confoederatus ('united by a league, confederate'), from con- ('together') + foederatus ('bound by treaty').

Examples:

1. The police arrested his confederate for assisting him in the robbery.
2. He acted as a confederate to the undercover agent in the sting operation.
3. The confederates believed in the right of states to secede from the Union.