The story
From the surnames of the three physicists who proposed this limitation on the long distance travel of energetic particles. From Greisen + Zatsepin + Kuzmin. Named after the American physicist Kenneth Greisen, and the Soviet physicists Georgiy Zatsepin, and Vadim Kuzmin. The Soviet and the American teams each discovered the limit separately in the 1960s, being independently computed in 1966.
Etymology from Wiktionary, CC BY-SA 4.0.