Nobody knows what the earliest word used by humans was. The general consensus is the Sumerians developed the first written language around 3500 BC, using wedge-shaped symbols called cuneiform. Scholars hypothesize that the first word was either “ouch” or “me,” although “pie” had to have been up there since the shapes surely reminded the Sumerians of it.
For the last 5,500 years humans have spent lifetimes searching for the best words to communicate. The precise word can make the difference between understanding and confusing people to the point of banging their heads against metal railings until blood drips out their ears. It doesn’t help that individual words can have many definitions, such as “gross,” which can mean… icky, large, 144 or the thought of Mitch McConnell naked.