Why is aces and eights the “dead man’s hand”?

Aces and eights was the hand held by Deputy U.S. Marshal James Butler (“Wild Bill”) Hickok when he was killed.

On August 2, 1876, in a saloon in Deadwood Gulch, Dakota Territory, Hickok was shot in the cheek by fellow poker player Jack McCall.

McCall later said he had killed Hickok for shooting his brother.