Why was General George Patton's nickname was old blood and guts?

Answer 1

Patton ran over people's bodies in the road gaining the nickname "old blood and guts"

He did not literally run over bodies. He did give a speech very similar to that depicted at the beginning of the movie "Patton" where he told his troops to spill the enemy's blood.

The nickname was pinned on him largely by his men for his bold, sometimes costly, plans: "His Guts and Our Blood"

Answer 2

His men called him that because he was a hard nosed, combative leader that took risks, often leading to many casualties. However, he inflicted many times the casualties on the Axis troops than he sustained among his own men.