To quote Edzard (2003),
Emesal is a sociolinguistic variety of Sumerian attested for the speech of women or goddesses.
In some Sumerian texts, women or goddesses have their speech transcribed in a way that doesn’t match “standard” Sumerian. Emesal uses the same grammatical structure, morphology and most of the same vocabulary as regular Sumerian, but some vocabulary is swapped out or pronounced differently. For example, the “standard” Sumerian dug “good, just, sweet” is pronounced zeeb in Emesal; dingir “deity” is dimmeer, and en “lord, priest” is umuun.
It’s clear this is meant to represent a variation on Sumerian spoken by women, and not a written variety (Enheduanna, a woman poet, does not write in Emesal) nor a separate language. It’s also not universal, as some women’s speech is written in Emesal and some is not. The Sumerians were aware of Emesal, but didn’t pay it enough mind to write much about it; the Akkadians only produced a couple of tablets explaining it, since they mostly cared about Sumerian legal documents and historical records, very few of which included Emesal.
The origin of the term “emesal” itself is from eme “tongue” plus sal, which has several meanings but probably in this case means “thin, narrow” or maybe “high-pitched”.