Idiom:  Word of Mouth;
used as a noun, usually as the object of a prepositional phrase.

First Example:  
“Before newspapers were invented, news was spread by word of mouth.”

Meaning:  Word of Mouth refers to information
that was obtained via gossip spread by spoken communication.  In this example, the primary method in which
information was disseminated was primarily through verbal communication.  In other words, before newspapers were
invented, people would find out about current events through gossip, rumors, or
other verbal forms.  This idiom can apply
to any situation where information can be obtained through verbal means.  This idiom is most often used as an object
for the prepositions, “by” or “through”.

Here is another example:

Second Example:
“The rumor that Cynthia had been cheating on her boyfriend
was spread through word of mouth.”

Meaning: In this case, the idiom, Word of Mouth, serves to express the idea that the rumor behind
Cynthia cheating on her boyfriend had been mainly transmitted via verbal
communication.  Whether or not there is
any truth to the rumor is irrelevant.  In
this example, the emphasis is that the rumor had been communicated orally
between different people.  In this
example, the idiom is being used as a noun.

This idiom is from the book “The Idiom Adventure –
Fluency in Speaking and Listening,” which is used as supplementary
material in LSI’s Intermediate Conversation classes.

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