What is the origin of the idiom "by word of mouth" and when to use it?

The idiom "by word of mouth" has ancient origins and reflects the traditional way information was passed from person to person in oral cultures before the widespread use of written language. It dates back to a time when people relied on spoken communication to convey news, stories, and knowledge. This expression is used to describe the transmission of information, rumors, stories, or knowledge through spoken communication rather than through written documents or formal channels.