to the marrow Definition

marrow
noun

1. The soft tissue that fills the internal cavities of bones.

Form: bone marrow (also)
2a. An annual plant with large prickly leaves, native to tropical America but cultivated worldwide for its large, oblong, edible fruit;

Form: vegetable marrow (also)
2b. The fruit of this plant which has a thick, green or striped skin, and soft white flesh, and is cooked as a vegetable.

Form: vegetable marrow (also)
3. The innermost, essential or best part of anything.

Thesaurus: essence, quintessence, heart, core, kernel, pith, quick, soul, nucleus.

Derivative: marrowy
adj

Idiom: to the marrow

Right through.

Etymology: Anglo-Saxon mærg.