The word is so interesting, and it's come up twice this week.
I asked Afsaneh how to say, "No problem" in Farsi. The answer, though I am surely transcribing it with mistakes, is something like "Moshkeli nadire" (MOSH-KAY-LEE NAD-DEE-RAY). This made easy sense, because both the words are familiar.
"Mafi moshkela" we used to say in Saudi, for "No problem."
"Nadir" is "rare" in Turkish.
Then, yesterday, Robin asked for the opposite of "zenith." He was fishing around for a word that he already knew. "Nadir," I put forward, "but it's an Arabic word, I think." "No, that's it!" And so it is. "Nadir" is English for the direction pointing directly below something. It can also work like "trough" or "bottom." Such words are almost invisible, they are so rarely used. Likewise, the spiderweb connections between languages often go unnoticed.
"Nadir" comes from the Arabic نَظِيرُ السَّمْت, nadhir as-samt, "counterpart to the zenith".