unguarded moment

Translations
неосторожный момент
He said something in an unguarded moment that he later regretted.
Он сказал что-то в неосторожный момент, о чём позже пожалел.
Definitions
An unguarded moment refers to a time when a person is not being cautious or defensive, often leading to vulnerability or the possibility of revealing true feelings, thoughts, or behaviors that they might typically conceal.