Idioms and Phrases with word of mouth, by
word of mouth, by
Orally, by one person telling another, as in They don't advertise; they get all their customers by word of mouth. [Mid-1500s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.