Innermost

wiktionary

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From Middle English innermost, innermest, in-nermast, alteration (due to Middle English inner, innere(“inner”)) of Old English innemest(“innermost”), equivalent to inner +‎ -most.

etymonline

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innermost (adj.)

mid-14c., from inner + -most. In the same sense innerest is from c. 1200. The older word is inmost. Innermore also existed in Middle English.