by word of mouth
Meaning: If something becomes well-known by word of mouth, it becomes well-known because people are telling each other about it, and not because of advertising or other marketing tools.
- Word of mouth made this movie a hit. A few people saw it and told their friends about it, and then they told more people about it, and before long everyone was going to see it.
- The best possible marketing tool is word of mouth, but a product has to be extremely good to earn it.