Ghiscari or Old Ghiscari is the language of the ancient Ghiscari Empire. It is still spoken in some parts of the Slaver's Bay region of Essos, more frequently by slaves than the noble caste, who prefer to use High Valyrian.
mhysa ("me-sar") - "Mother".
Speakers of Ghiscari
In the books
In the A Song of Ice and Fire novels, Ghiscari is much more widely-spoken in Slaver's Bay (as part of an attempt to reclaim their cultural heritage and erase all trace of the Valyrian conquest) and remains the primary tongue of the kingdom of Ghiscar to the south. When Daenerys asks what language is spoken is Yunkai, Missandei says that they speak a version of Low Valyrian. The cities of Slaver's bay actually have a polyglot population, because slaves are brought in from distant areas. In the books, Daenerys is hailed by the freed slaves as "mother", but using the different words for "mother" in at least five different languages: Mhysa, Maela, Aelalla, Qathei, and Tato. "Mhysa" is the word for "mother" in Old Ghiscari, but the other four languages aren't identified. The TV series simply had the freed slaves call her only "Mhysa" and omitted the other forms.
The books aren't very consistent on this point, because at times Old Ghiscari is spoken of as if it is functionally a dead language, while at other times it appears that at least some of the population speaks it.
For the TV show, the Ghiscari language appears to be regarded as more of a dead language (hence the "old" appellation) than an active one and the people of Slaver's Bay instead speak a variant of High Valyrian with some old Ghiscari language influences. For the TV show, it is not yet known if the Ghiscari kingdom to the south speaks Ghiscari or a similar variant of High Valyrian.