zenith "point of the sky directly overhead" is from late Middle English cenyth, senith, cinit, from Old French cenit (Modern French zénith), from medieval Latin cenit. The Latin word is from Arabic samt, a component of samt arrās (سمت الرأس) "path over the head". samt "way" + al "the" + rās "head".
Why did samt get borrowed into Latin as cenit? Maybe scribal error was involved. The OED says the derivation is obscure. Also, it seems that the modern English word has influenced the modern French word, or vice versa, since they are both spelled the same.
Arabic samt was borrowed from Latin sēmita "sidetrack, side, path", which is a combination of sē- "apart" (from Proto-Indo-European *s(w)e- the third person pronoun) plus Proto-Indo-European *mi-tā, a form of *mei- "to change, go, move" (also see here).
*mei- in its extended form *h₂meigʷ- became Greek ἀμοιβή amoibē "variation", and amoeba.
Another cognate is mad, from Old English *ȝemǣdan "to make insane or foolish" from Proto-Germanic ǥa-maiđ-az "changed (for the worse); abnormal" from the suffixed o-grade form *moi-to- (*-to- formed adjectives of accomplishment; it is found in the English adjectival suffix -ed, as in bearded).