Zenith \ZEE-nith\ , noun;
1. A highest point or state; culmination
2. The point on the celestial sphere vertically above a given position or observer. Opposite of nadir.
This word dates to the late 14th century and comes from Old French cenith (Modern French zénith). It's origin is rooted in Middle Latin cenit (also senit), which is a bungled scribal transliteration of Arabic samt ("road, path"). Samt is an abbreviation of samt ar-ras, which is literally "the way over the head." The bungling is probably partially influenced by the classical Latin semita ("sidetrack, side path" - notion of "thing going off to the side"). Semita is a compound of se- ("apart") + *mi-ta (suffixed form of Proto-Indo-European base *mei meaning "to change").
Nadir also dates to the late 14th century and comes from Middle Latin. The Arabic word from which it derives is nazir, which means "opposite to." The association between nadir and zenith is actually a bit of a mistake. In Arabic, "zenith" was as-samt and "opposite of zenith" was nazir as-samt, so when this word was adopted into Middle Latin it was abbreviated erroneously and became nadir.
Today's word and the first definition were both taken from Dictionary.com's 'Word of the Day' for Thursday, December 23
Etymologies come from the Oxford English Dictionary and/or Etymonline.com