hindmost (adj.)

c. 1300, "pertaining to the rear, back, posterior," perhaps a back-formation from Old English behindan "back, behind," used as adverb and preposition (see behind), or from or influenced by Old English hindan (adv.) "from behind," from Proto-Germanic *hind- "behind" (cognate with Gothic hindan (prep.) "on that side of, beyond, behind;" German hinten "behind"), of unknown origin. Possibly influenced by Middle English hiner (adv.) "back, rear."

superlative suffix of adjectives and adverbs, Middle English alteration (by influence of unrelated most) of Old English -mest, a double superlative, from -mo, -ma (cognate with Latin -mus; compare Old English forma "first," meduma "midmost") + superlative ending -est. Now generally mistaken as a suffixal form of most.