'zenith' originates from Medieval Latin/Old Spanish and ultimately from Arabic. It entered English via Medieval Latin/Old Spanish forms such as 'cenit' or 'zenit', which came from Arabic 'samt' (often in the phrase 'samt al-ras').

'samt' (Arabic) → Medieval Latin/Old Spanish 'cenit'/'zenit' → Middle English 'zenith' (modern English 'zenith').

Initially it referred to a 'direction' or 'path' (Arabic sense of 'samt' as direction, especially 'direction of the head' in the phrase). Over time it became specialized in astronomy to mean the point directly overhead and then generalized to mean the highest point or culmination.