In the Torah, we are commanded to take פרי עץ הדר - the fruit of the hadar tree. However, today we all know this fruit by a different name - אתרוג etrog (or ethrog or esrog). What is the etymology of this name?
a loan word from Persian turnuj, whence also Arabic turunj, utrunj, utrujj
He adds that this is the root of the Spanish word toronja, meaning "grapefruit".
Mike Gerver elaborates on Klein in this Mail-Jewish discussion:
Etrog, on the other hand, is listed in the same book as borrowed from Persian turung or Mandaic trunga. (The form "etrunga" is found in Kiddushin 70a.) The Persian word, according to Chaim Rabin's article "Lexical Borrowings from Indian Languages as Carriers of Ideas and Technical Concepts" (in "Between Jerusalem and Benares: Comparative Studies in Judaism and Hinduism", page 25, edited by Hananya Goodman, SUNY Press) comes from Tamil, and is related to "matulankam" and "matulai" which mean pomegranate or lemon. (In modern Tamil, pomegranate is "matulanpazham," where "pazham" means ripe fruit.) Rabin says that there is no similar word in Sanskrit, suggesting that etrogs were originally found only in southern India where Tamil and other Dravidian languages are spoken, and only spread to northern India and Persia in a later period (after Sanskrit). I'm not sure what this implies about the question of whether "pri etz hadar" always meant only the etrog, and whether the "etz hadaat" could have been an etrog. It is quite possible, of course, that "trunga" did not mean an etrog, but a different kind of fruit, at the time the word was borrowed from Dravidian, and that it was this other fruit that was only found in southern India. The "kam" at the end of "matulankam" (and hence the "nga" at the end of "trunga") are presumably related to "kaay" meaning "fruit" in modern Tamil. The same root is apparently found in the Persian word "naranga" (source of "naranja" in Spanish and hence "orange" in French and English), which was also borrowed from a Dravidian language. In modern Tamil, "naru" means "smelly," so "naranga" could mean "fragrant fruit." (Words that mean "fragrant" tend to evolve to mean "smelly" in any language.) Oranges are thought to have come to the Middle East and Europe from northern India, according to the Encyclopedia Brittanica, and to there from southern China and Indochina, so the question arises as to why the word would be borrowed from a Dravidian language. One possibility is that the word dates back to the period before the Indo-European conquest of India, when Dravidian languages were spoken in Northern India as well. So the "g" in etrog would be cognate with the "g" in orange.
The Gemara that Mike quotes is a fascinating one for those interested in language. It contains a dialogue between Rav Nachman and Rav Yehuda, where Rav Nachman insists on using "fancier" words, instead of the more commonly used ones. For example, Rav Nachman calls a fence a גונדריתא gundarita. Rav Yehuda asks, why couldn't you use the Biblical מעקה ma'akeh or the Rabbinic מחיצה mechitza? Similarly, Rav Nachman uses the term etrunga אתרונגא , and Rav Yehuda accuses him of being snobby for not using either the Rabbinic etrog or the common Aramaic etroga אתרוגא. Whatever your opinion of Rav Nachman's linguistic approach (and I'm sure many of us know people who like to do the same), he certainly seemed to use a word closer to the original Persian. And in the Yerushalmi (Gittin chapter 3, and Sukkah chapter 3) actually uses the term תרונגא trunga.