by word of mouth

Từ điển WordNet v3.1 - WordNet Dictionary

adverb

1. orally (Freq. 1)
- I heard it viva voce
• Syn:
viva voce
2. by spoken rather than written means
- these stories were transmitted by word of mouth
• Syn:
orally
• Derived from adjective:
oral (for: orally)