It’s hard to say exactly when the user was born, but it might be Don Norman at Apple in 1993 (referenced by Peter MerholzPeter Merholz. "Whither "User Experience"?". peterme.com. (1998): [http://www.peterme.com/index112498.html] ):“
The term “user” has been around in architecture and design at least since the 60s, if not earlier (Eliot Noyes was talking about users relating to IBM product design in the 50s). Avigail Sachs has a killer piece called “Architects, Users, and the Social Sciences in postwar America” that seems especially relevant here - she shows how the idea of the “user” was a central piece of the strong relationship between design and systems-oriented social science in the 60s and 70s, until the early 80s. It was especially present in community-oriented participatory design, which was happening at MIT in that period (and, of course, contributed heavily to the founding of a certain lab at MIT…)
The essay can be found (in full) in the collection “Use Matters”.