a great person or group has failed or been disgraced
—Two years ago the president and vice president were celebrating their reelection. Now they are in prison. How the mighty have fallen.

Throughout his rule as king of Israel, Saul fought against the Philistines, and in the end, he was gravely wounded in a battle with them. Rather than have his enemies find him and kill him, Saul killed himself:

Saul said to his armor bearer “Draw your sword and stab me with it! Otherwise these uncircumcised people will come, stab me, and torture me.” But his armor bearer refused to do it, because he was very afraid. So Saul took his sword and fell on it. When his armor bearer saw that Saul was dead he also fell on his own sword and died with him. So Saul, his three sons, his armor bearer, and all his men died together that day. (1 Samuel 31:4-6)

Upon hearing the news that Saul and his son Jonathan were dead, David was filled with grief. He wrote a song in their honor, which begins,

The beauty of Israel lies slain on your high places!

How the mighty have fallen [how are the mighty fallen—KJV]! (2 Samuel 1:19)

About the deaths of Saul and his armor bearer, the Wycliffe Bible—the Bible’s first English translation, completed in the late 1300s—says that “Saul took his sword, and felled thereon,” and the armor bearer, “also he felled upon his sword.”

Today, to fall on one’s sword means to take the blame for a mistake or wrongdoing and to punish oneself, often by resigning from a job or position.