In the early medieval world, a man’s word, honour and reputation were vitally important – more valuable than life. Breaking an oath carried incredibly severe consequences. An oath-breaker was about the lowest form of scum imaginable. Nobody would want to be associated with an oath-breaker, who would probably lose the protection of his lord (though perhaps not his kindred), making him extremely vulnerable to his enemies.
Anyone bringing a serious accusation (together with any witnesses) would be prepared to take an oath that they are speaking the truth and have no malicious intent. This was not simply making a statement lightly: a number of oath-helpers were required. Oath-helpers had to be free men of similar/better status. Historically, six or twelve oath-helpers were normal.
The catch was that the oath-helpers were putting their reputation on the line – effectively swearing that they believe the defendant is innocent.
A man who was not oath-worthy (ON nithing) could neither swear an oath or act as an oath-helper. If he tries to, he should be thrown out of the court and probably fined (if he’s lucky!).
Credits: with thanks to Benedict Coffin