The word 心底 is formed by combining the character for "heart" (心) with the character for "bottom" or "base" (底), creating a powerful spatial metaphor. It conceptually visualizes the heart as a container or a deep place, with 底 representing its innermost, furthest recesses. Thus, the compound doesn't just mean "heart", but specifically "the bottom of one's heart", evoking the idea of one's deepest, most private and genuine inner realm where true feelings and thoughts are stored away from the surface.