deep down

Adverb

idiomatic not-comparable

(idiomatic) Fundamentally; in essence; in reality

Japanese Meaning

本質的に / 根本的に / 実際は / 心の奥底で

She smiled and chatted politely, but deep down she was furious.

彼女は笑顔で礼儀正しく会話していたが、心の奥では激怒していた。

Sense（1）

(idiomatic) Fundamentally; in essence; in reality

He seems like an average businessman, but deep down he's an overgrown kid with a necktie.