in deep reflection

Translations
в глубоком размышлении
He was in deep reflection about his future.
Он был в глубоком размышлении о своем будущем.
Definitions
A state of serious and thoughtful consideration or contemplation, often involving introspection and a thorough examination of one's thoughts, feelings, or experiences.
Words
reflection

noun
отражение
deep

adjective
глубокий