In North American old west legend, Widowmaker is the name of Pecos Bill's white horse.

Pecos Bill tamed the wild mustang mare when he was still a child living with coyotes. When he got married to his sweetheart, Slewfoot Sue, she demanded to ride Widowmaker, who bucked her up to the moon.

A heavy metal can device with handles on the side used to drive fence posts into the ground. So called because the effort it took to use the device would quite likely kill you.

A widowmaker is also a falling branch from a burning or smouldering tree.
Or a branch in a tree that has been weakend by a recent fire and falls on someone's head.

Wildlands firefighter jargon.

Widowmaker is the nickname given to a heart attack caused by a blood clot in the left anterior descending artery. It is caused by plaque build up in the artery, and may have virtually no symptoms until the victim falls dead suddenly and unexpectedly.