compurgator

Merriam-Webster's Word of the Day for April 14, 2011 is:

compurgator • \KAHM-per-gay-ter\  • noun
: one who under oath vouches for the character or conduct of an accused person

Examples:
As a compurgator, you do not have to believe in the innocence of the defendant, but you do have to feel confident speaking positively about that person's character.

To clear himself, the defendant required corroboration from a prescribed number of compurgators or 'oath-helpers,' which varied according to the nature and severity of the accusation. Neither the defendant nor his compurgators were required to present any evidence to the court. -- From Bruce L. Benson and Paul R. Zimmerman's 2010 book Handbook on the Economics of Crime

Did you know?
Compurgator is a descendant of the Latin verb compurgare, meaning to purify wholly. The root of that word, purgare, also gave English purge (to clear of guilt, to cause evacuation from, or to get rid of) and expurgate (to cleanse of something morally harmful, offensive, or erroneous). Compurgator has occasionally been used in a more general sense of one who supports or defends another, but its primary application is to the specific legal situation in which someone appears in court as a character witness for the defendant. Compurgator has been used in English with this specific legal meaning since the 16th century.