Located farthest inside; deepest or most hidden, especially referring to private thoughts or feelings.
From 'inner' (Old English 'inne' meaning 'within') combined with the superlative suffix '-most' (from Old English 'mest' meaning 'most'). The compound has meant 'deepest inside' since medieval times.
Superlatives like 'innermost,' 'topmost,' and 'foremost' are grammatical fossils—we barely add '-most' to new words anymore because 'most' became a separate word, so these frozen forms are linguistic time capsules from Old English.
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