Dr. Winston Royce in 1970 described the “waterfall” lifecycle in a paper he wrote for the IEEE WESCON (http://www-scf.usc.edu/~csci201/lectures/Lecture11/royce1970.pdf). It was not called “waterfall” in that paper, but the image, as shown in the paper, suggested that to many people. It may be that the term was first used in this paper several years later: https://static.aminer.org/pdf/PDF/000/361/405/software_requirements_are_they_really_a_problem.pdf.

When I was working during the 70’s and definitely in the late 80’s into the 90’s in the Bell System, the term “waterfall” was well-known and commonly used to describe a phased, sequential way of working as Royce described. So it was not invented or popularized by those promoting Agile methods. By that time, it was not used much by those practicing it perhaps because it was just the way things were done.

Many frameworks for waterfall had different titles (just as they do for Agile frameworks today) which is how people referred to them (just as people say they are “doing Scrum” which means Agile to them).