Origin of the name of the Euphrates and Tigris rivers
It is believed that the name is very old, originating before the Sumerians, and most likely it is a linguistic heritage from an unknown people who may have preceded the Sumerians in settling the alluvial plain, and nothing is known about them except for the few linguistic traces they left in the names of some cities and professions, including the names of the Tigris and Euphrates.
The name of the Euphrates was mentioned in the cuneiform texts with a group of signs pronounced in the form of (Buranen) or (Burunna), and in the Akkadian language this is synonymous with the word (Burati) or (Buratam), and from this word the Arabic name (Furat) appeared.
It was said about the meaning of this name that it is a branch, tributary, or fresh water, and this is the meaning given by Arab linguists to the name of the Euphrates as well.
So we say Euphrates water, meaning fresh water.
As for the name Tigris, it appeared in the Sumerian language in the form (Adkna), and from it it was transferred to the Akkadian language and became pronounced (Deglet), and perhaps from it the Arabic name (Tigris) was derived.
Source
Introduction to the history of ancient civilizations
Dr. Taha Baqir