nadir

Nadir comes from Medieval Latin nadir, borrowed from Arabic naẓīr as-samt, meaning "counterpart of the zenith." The phrase combined naẓīr, "counterpart, corresponding to," with as-samt, "the zenith."

The facts don't describe a shift in meaning — the word entered Medieval Latin already carrying its sense of the point directly opposite the zenith.

Its origin ties it directly to the concept of the zenith, since Arabic built the term as the zenith's exact counterpart.