innermost
adj.the most inside or closest to the center of something. You use this to describe the part that is hidden or protected by other layers.
adj.the most internal or central part of a structure or group. Often used to describe the deepest layer of a physical object or the most private aspect of a person's thoughts.
“The innermost part of the tree is very hard.”
“The explorers reached the innermost chamber of the cave, where the air was thick with ancient dust.”
“The poet's innermost fears were revealed in the final stanza, where he confessed his deepest insecurities to the reader.”
“Mercury is the least studied of the four rocky, innermost planets in our solar system, which include Venus, Earth and Mars.”
From Middle English innermost, innermest, in-nermast, alteration (due to Middle English inner, innere (“inner”)) of Old English innemest (“innermost”), equivalent to inner + -most.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.