means The name you choose to identify yourself on a computer system, website, or service when logging in.

from A straightforward compound of "user" and "name," both plain English words. "User" comes from "use," rooted in the Latin "uti" (to use, employ), which arrived in English through Old French "user." The whole word is a child of the computing age — coined as time-sharing systems in the mid-20th century needed a way to tell one human apart from another, so each person got a "name" by which the machine could know its "user."