to the core; to the bone; to the marrow
Idiomatic expression meaning thoroughly or completely, often used with verbs like しみ込む, 愛する, or 腐る to emphasize deep penetration or intensity.
寒さが骨の髄までしみ込んだ。
The cold penetrated me to the bone.
彼は骨の髄まで政治家だ。
He is a politician to the core.