The widow maker is a nickname for a heart attack involving a blockage in the left anterior descending artery. This is the main artery on the front wall of the heart.
The term stems from the low survival rates of patients who experience heart attacks because of blockages of the LAD. The heart muscle must have ample blood supply for a person to survive. When the LAD becomes blocked, the heart is not able to pump blood through that artery. Since the LAD supplies blood to the entire front of the heart, a total blockage of that artery is often fatal.