Leshon Hakodesh, (also pronounced L'shon Hakodesh), or more commonly Lashon Hakodesh (Hebrew: לשון הקודש; lit. "the Holy Tongue"), is a Jewish term and appellation attributed to the Hebrew language, or sometimes to a mix of Hebrew and Aramaic, in which its religious texts and prayers were written, and served, during the Medieval Hebrew era, for religious purposes, liturgy and Halakha - in contrary to the secular tongue, which served for the routine daily needs, such as the Yiddish language.
Origins in the Classical texts
The phrase's first appearance is already in the Mishnah:
In its narrow sense, Lashon Hakodesh refers not to the Hebrew language in its entirety, but rather to the Biblical Hebrew only. In its broader sense, it was used for combining Hebrew and Talmudic-Aramaic within the Rabbinic Hebrew, which served the purpose of writing the Jewish classical texts of the Middle Ages and the Early modern period.
The exact meaning of the phrase "Lashon Hakodesh" becomes clear due to its contrary term. In the Mishnah and the Gemara the term was aimed to take out the foreign languages that were commonly spoken among the Jewish communities:
"For Rabbi said: Why use the Syrian language in the land of Israel? Either use the Holy Tongue or Greek! And R. Joseph said: Why use the Syrian language in Babylon? Either use the Holy Tongue or Persian!"
"Rabbi Hanina said: Because [the Babylonian] language is akin to the Lashon Hakodesh"
The Rishonim sages perceived only Biblical Hebrew, and not the Mishnaic Hebrew, as "Lashon Hakodesh". In Yiddish, the term "Loshn Koydesh" serves to describe its own Hebrew-Aramaic component, as opposed to words originating from German or Slavic languages. In some Jewish Haredi denominations, the term is meant to describe old Hebrew as opposed to Modern Israeli Hebrew, and few extreme Haredi denominations even try to avoid using renewed words since the Revival of the Hebrew language.
Jewish philosophers have offered various reasonings for Hebrew being the "Sacred Language". Nahmanides provided his reasoning, based on the way the Hebrew was being used:
"As I see it, the reason for the Rabbis calling the language of the Torah the Holy Tongue is that the words of the Torah and of the prophecies and all sacred utterances were all spoken in that language; it is the language that the Holy One, blessed be He, speaks with His prophets and with His people, saying, "I am ...," "Thou shalt not have ..." and the remaining commandments and prophecies; it is the language by which He is called in His sacred names... and in which He created His universe, gave names to heaven and earth and all therein, giving his angels and his host names — Michael, Gabriel, etc. — all in that language, and in that language naming the saintly people in the Land, such as Abraham, Isaac, and Solomon."
" I have also a reason and cause for calling our language the holy language-do not think it is exaggeration or error on my part, it is perfectly correct—the Hebrew language has no special name for the organ of generation in females or in males, nor for the act of generation itself, nor for semen, nor for secretion. The Hebrew has no original expressions for these things, and only describes them in figurative language and by way of hints, as if to indicate thereby that these things should not be mentioned, and should therefore have no names; we ought to be silent about them, and when we are compelled to mention them, we must manage to employ for that purpose some suitable expressions, although these are generally used in a different sense."— The Guide for the Perplexed 
- M. Friedlander (1904).  The Guide for the Perplexed (Friedlander), Part III, Chapter 8.
- Y. Frank & E.Z. Melamed (1991). Practical Talmud Dictionary. Feldheim Publishers. ISBN 978-0873065887.
- R. C. Klein (2014). Lashon HaKodesh: History, Holiness, & Hebrew. Mosaica Press. ISBN 978-1937887360.
- D. Leitner (2007). Understanding the Alef Beis. Feldheim Publishers. ISBN 978-1598260106.
- M. Munk (1986). The Wisdom of the Hebrew Alphabet. Artscroll/Mesorah Publications. ISBN 978-0899061931.