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by word of mouth (orally, from person to person by the spoken word)
1. по устной рекомендации, со слов других людей
2. [из уст в уста]

I learned that there would be a holiday next week by word of mouth. — ...узнал от людей/со слов других людей (=heard it on the grapevine)

He never advertises. He gets all his clients by word of mouth. — ... все клиенты приходят к нему по устной рекомендации других людей.

[сарафанное радио ]
[on the grapevine]