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1.
n. Word of mouth (see also 'Word of Mouf'): a spoken and sometimes secret language amongst a culture, clique, or team, or world.

Explanation: The reason why ideas get popular, cool, funny, interesting, or trendy very rapidly. Information can be passed along very quickly now a days with the age of the internet.
a business idea, fashion trend, virus, epidemic, viral post, Kanye West, McDonald's, Apple, social media, a play-on-words, nickname, handshake, a lifestyle

1.) Will: Steve Jobs' and Apple combine amazing customer service, quality, and the power of Word of Mouth for success in their business.

2.)

Marcus: How did the whole school already find out me and her are dating?
Jonny: Word of Mouth.
by McDiddy July 22, 2014