As a military term. There use to be people known ar archers - those who shot arrows at the opposing army. Archers kept a supply of arrows, some of these were the finest iron tipped shafts made by a real fletcher with real feathers to provide angular torque to keep them straight in flight (these were the best). Most of the arrows in an archers quiver however were the mass produced, low quality shafts, some just twigs broken off of near-by trees. When the archers started shooting they would use the low quality arrows first as the opposing army was still far away and the possibility of actually hitting anyone was low. As the opposing army got nearer however (as in a charge) the archer got a little more concerned about their own safety and began using the better arrows to ensure better accuracy and kill power. Therefore master archers were in the habit of instructing their students 'save the best for last.'