"situated in the rear, pertaining to the rear, toward the back," late 14c., probably from an unrecorded Old English adjective from hinder (adv.) "behind, back, afterward," but treated as a comparative of hind (adj.). Related to Old High German hintar, German hinter, Gothic hindar "behind" (prep.).
Middle English had hinderhede, literally "hinder-hood; posterity in time, inferiority in rank;" and hinderling "person fallen from moral or social respectability, wretch," from an Old English term of contempt for a person devoid of honor. Also compare Scottish hinderlins "the buttocks."
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.
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Harper, D. (n.d.). Etymology of hindermost. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/hindermost