The lowest point, opposite of zenith.
Dated back to the late 14th century, this noun is an astronomy related one. Although Mid Latin records too display nadir, this word originates from the Arabic nazir, literally meaning ‘opposite’. The original Arabic astronomy term is nazir as-samt, meaning opposite of the zenith (with as-samt being zenith).
For 300 years this word was strictly used with astronomy related records, this however changed in 1793 with the first record to this word being used as the lowest point of anything.
Why this word?
Both nadir and zenith are interesting perspectives in astronomy and outside it. In the celestial sphere, both points are directly below (nadir) or above (zenith) the observer and thus very easily seen by him. The borrowed sense of the lowest point, provides us with a sad point of view as one cannot discard his lowest point while seeing it so vividly…
How to use nadir?
Nadir is a simple noun, representing the lowest point [of something], you can use it simply as such in any context.
“The company’s revenues hit nadir in 2012 Q2”
“This last year was my life’s nadir”