In vino veritas.
~ Latin proverb
Literal translation: “In wine there is truth.”
English equivalent: “There is truth in wine.”
It’s probably one of the oldest sayings about wine. The Latin phrase appear in the Naturalis Historia of Pliny the Elder. There is a similar expression in Greek “Ἐν οἴνῳ ἀλήθεια” (En oino álétheia), which is traced back to a poem by Alcaeus. The Latin expression, together with its counterpart in Greek is found in Erasmus’ Adagia, I.vii.17.
When people are drunk they lose their inhibitions which makes them express their true thoughts and feelings. Additionally, drink makes people more honest and less likely to hold back out of fear of the consequences.