Orally, by one person telling another, as in They don't advertise; they get all their customers by word of mouth. [Mid-1500s], see WORD OF MOUTH.
American Heritage Idioms
Phrase(s): by word of mouth
by speaking rather than writing. • I learned about it by word of mouth. • I need it in writing. I don’t trust things I hear about by word of mouth.
McGrawhill's American Idioms And Phrasal Verbs