inmost

Etymology

From Middle English inmost, from Old English innemest, a double superlative form from inne (“within”), from in (“in”). The modern form is due to confusion with most.

adj

1. The very deepest within; farthest from the surface or external part; innermost
   Virginia Carteret was finding it a new and singular experience to have a man tell her baldly at their first meeting that he had read her inmost thought of him. 1905, Francis Lynde, A Fool for Love, The Bobbs-Merrill Company, Indianapolis, page 25

noun

1. That which is innermost; the core.