deepest part; innermost depths
Refers to the deepest physical or metaphorical part of something, such as the ocean, a cave, a forest, or one's heart.
The deepest part of the lake has not yet been explored.
In the innermost depths of his heart, there was a secret he couldn't tell anyone.
Compound of 最 (sai, 'most') + 深 (shin, 'deep') + 部 (bu, 'part'). The reading is on'yomi throughout.