What is the allusion to "shooting a tiger with an umbrella" and what is its meaning?

A 93-year-old man married a young wife. Soon, he told the doctor that his wife was pregnant.

The doctor said, "Let me tell you a story. An idiot went hunting and accidentally took an umbrella as a gun. Suddenly a tiger jumped out and pounced on him. He raised his umbrella and killed the tiger. "

"This is impossible!" The old man cried, "It must be someone else!"

"Yes," replied the doctor, "I think so."

Google found that "for a time, it was widely rumored in academic circles that Yang Gong was impotent and tried to kill the tiger with an umbrella", which was a story that mocked Yang Weng's love.