A new language created by the Blood Elves to seperate themselves from their past, and to help prevent alliance(which still includes High Elves and Half-elves) from learning vital information about their plans. Even though the Blood Elves were established less than ten years before World of Warcraft, it is not identical to the far older High Elven language, Thalassian.
Origin of the name
The most logical theory is that the blood elves changed the name of their former language (thalassian), into something that fits into their new name, Sin'Dorei.
Template:Stub/Accuracy The word Sindassi likely draws it's root from the word Sindarin, the language of the Elves of Beleriand in Tolkien's Silmarillion. Sindarin was the most common language of the Moriquendi, the division of elves that never dwelled in the blessed realm of Valinor. The Ñoldor, after returning to Middle Earth, used Sindarin since it was the prevailing tongue among most of the elves in Beleriand when they arrived.