Old English beneoðan "under, below, in a lower place, further down than," in late Old English "lower in rank, degree, excellence, etc.," from be- "by" + neoðan "below, down, from below," from Proto-Germanic *niþar "lower, farther down, down" (see nether). Meaning "unworthy of" is attested from 1849 (purists prefer below in this sense). "The be- gave or emphasized the notion of 'where,' excluding that of 'whence' pertaining to the simple niðan" [OED].
<a href="https://www.etymonline.com/word/neath">Etymology of neath by etymonline</a>
Harper, D. (n.d.). Etymology of neath. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/neath