a person wants to do something, but he lacks the skill, energy, or strength to follow through
—I’d love to play tennis with you, but my 60-year-old body doesn’t agree. I guess the spirit is willing, but the flesh is weak.

The Jewish authorities knew that Jesus said that they deserved God’s judgment. The more they heard him the more angry they became, and they wanted to arrest Jesus and kill him. Knowing that the Jewish leaders would succeed and that he would die soon, Jesus went with his disciples to the Garden of Gethsemane to pray:

Then Jesus went with them to a place called Gethsemane, and he said to the disciples, “Sit here while I go over there and pray.” He took with him Peter and the two sons of Zebedee, and became anguished and distressed. Then he said to them, “My soul is deeply grieved, even to the point of death. Remain here and stay awake with me.” Going a little farther, he threw himself down with his face to the ground and prayed, “My Father, if possible, let this cup pass from me! Yet not what I will, but what you will.” Then he came to the disciples and found them sleeping. He said to Peter, “So, couldn’t you stay awake with me for one hour? Stay awake and pray that you will not fall into temptation. The spirit is willing, but the flesh is weak [the spirit indeed is willing, but the flesh is weak].” (Matthew 26:36-41)

In modern English, gethsemane can mean “a time or place of great suffering.”