loosen (one's) tongue

Translations
развязать (кому-то) язык
A few drinks were enough to loosen his tongue and he started revealing secrets.
Несколько выпивок были достаточно, чтобы развязать ему язык, и он начал раскрывать секреты.
Definitions
To make someone more willing to speak freely or reveal information, often by making them feel relaxed or by giving them alcohol.