This phrase is used to indicate that something will never happen or is highly unlikely. It is a humorous reference to Christopher Columbus (Colón in Spanish) and his statue in Barcelona where he is pointing with his finger, implying that when he lowers his finger, then the unlikely event will happen.
Examples
- Spanish: Te casarás con un príncipe cuando Colón baje el dedo.
- English: You will marry a prince when Columbus lowers his finger.