The line comes from the HBO comedy Eastbound & Down, where Ashley Schaeffer, played by Will Ferrell, uses plums as a crude stand-in for gut instinct and swelling confidence. Clipped from the show, the quote spread through forums, reaction videos and soundboards because the phrase works even without context. Its short length and serious delivery made it easy to reuse for predictions, bets and hunches, and it remains popular as a shorthand for being absolutely certain about something you cannot actually prove to anyone watching online today.