What word does your mother always say?
late 14c., from Old French cenith (Modern French zénith), from Medieval Latin cenit, senit, bungled scribal transliterations of Arabic samt "road, path," abbreviation of samt ar-ras, literally "the way over the head." Letter -m- misread as -ni-.
The Medieval Latin word could as well be influenced by the rough agreement of the Arabic term with classical Latin semita "sidetrack, side path" (notion of "thing going off to the side"), from se- "apart" + *mi-ta-, suffixed zero-grade form of PIE root *mei- "to change" (see mutable).
The point on the celestial sphere that is directly above the observer (90 degrees above the celestial horizon). Compare nadir.