The story
From Serena (“Serena Williams”) + slam (“grand slam”). From Serena Williams who achieved the feat (though she was not the first) and was greatly celebrated for it.
Etymology from Wiktionary, CC BY-SA 4.0.
Serena Slam
Today it means: A type of grand slam in tennis, where a player is reigning champion of all the major tournaments at the same time, but not all the tournaments have occurred in the same tennis season/calendar year. Full definition →