Having Heart is simply another way of ascribing some supernatural power to an achievement that was difficult or seen as beyond the capability of the individual.
The saying became popular after the race horse [Seattle Slew](https://en.m.wikipedia.org/wiki/Seattle_Slew) passed away and the autopsy showed his heart was twice the size of any other horse seen. Seattle Slew retired as undefeated and a triple crown winner. He was the first horse in history to achieve that distinction.
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