The phrase comes from Conor McGregor, the Irish UFC star known for trash talk as much as fighting, who told an opponent at a press conference that signing to fight him was cause for celebration at home because it meant the biggest payday of their career. The red panty night idea spread through MMA highlights, interviews and remixes, then became shorthand online for any moment when money suddenly arrives. It remains popular in sports edits and reaction videos because the line explains its own joke.