What exactly does Boomer Sooner mean?

Answer 1

The "boomers" were a group (most notably David L. Payne, for whom Payne County, OK is named) who agitated for more than ten years before 1889 to open the unassigned Indian lands in Oklahoma to settlement. The "sooners" were those who illegally entered the territory before the opening gun at high noon on April 22, 1889 in order to stake claims to the most desirable lands. Payne did not live to see his dream realized. He died on November 28, 1884.