Every barber knows that
“Omnibus notum tonsoribus.” Horace: 1 Satires, VII. 3.
In Rome the tonstrinæ or barbers' shops were the fashionable resort of loungers and idlers. Here every scandal was known, and all the talk of the town was repeated.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894