to the backbone

Prepositional phrase

idiomatic

(idiomatic) Through and through; thoroughly; entirely.

Japanese Meaning

徹底的に / 完全に / 心底から

He was honest to the backbone, refusing to compromise his principles for personal gain.

彼は骨の髄まで正直で、私利のために自分の信念を曲げることを拒んだ。

Sense（1）

(idiomatic) Through and through; thoroughly; entirely.

In temperament he was Dutch to the backbone — at least as we imagine Dutch.