Old English niþera, neoþera "down, downwards, lower, below, beneath," from Proto-Germanic *nitheraz (source also of Old Saxon nithar, Old Norse niðr, which contributed to the English word, Old Frisian nither, Dutch neder, German nieder), from comparative of PIE *ni- "down, below" (source also of Sanskrit ni "down," nitaram "downward," Greek neiothen "from below," Old Church Slavonic nizŭ "low, down").
Also an adverb in Old English and Middle English. It has been replaced in most senses by lower (adj.). Of countries, "situated on lower ground" (late 14c.). In Middle English (and after) used also of body parts.
Absolon hath kist hir nether eye. [Chaucer, "Miller's Tale"]
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 nethermost. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/nethermost