Leshon Hakodesh

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.

The phrase's first appearance is already in the Mishnah:

"The following may be recited in any language: The Torah-portion of 'Sotah', the confession made at the presentation of the tithe, the 'Shema', and the 'Prayer' …

The following are recited in the Holy Tongue: The declaration made at the 'First Fruits', the formula of 'Halizah', the blessings and curses, the benediction of the high priests …"

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:

...
Wikipedia