If a teacher or a foreman tells someone it’s time to buckle down, they mean “Quit fooling around, this is serious business,” and they’re using an expression from the days of knighthood.
When preparing for combat, knights required their squires to attend to their armor by oiling it, laying it out, and then buckling it onto their masters’ bodies.
How well this was done could be the difference between life and death for the knight, so buckling down was very serious business.
