Well first of all, I couldn't find any Spanish or Mexican slang dictionaries online that listed higado as a term. So then of course I wondered if I had been misled all along into thinking this was a common colloquial phrase, when only my family uses it. This has happened to me before. Yes, it has made me a little calloused. And bitter. Jaja.
Putting that thought aside and assuming some people use it outside of my family, I continued my quest.
The etymology of higado comes from the Latin ficatum meaning liver.
From ficatum, we got:
French - foie
Italian - fegato
Portuguese - figado
Spanish - higado
That kind of left me stuck so I looked into the etymology of liver in English. I came across Easton's 1897 Bible dictionary. This might be a bit of a stretch--but I couldn't come up with any other reasoning:
The Hebrew word for liver is kabhed also meaning heavy--appropriate because the liver is the heaviest of the viscera. Because of it's weight, it Biblically achieved sacred status, being burned upon the altar, and not used as sacrificial food.
In Ezekiel, the King of Babylon "looked upon the liver" to one of the most ancient of all modes of divination.
Maybe--just maybe--this notion of the liver's divine superiority got translated through the times to be associated with the self, and the self's center.
Or maybe even because of it's actual weight, it became associated with the self's center.
MAYBE I should ask some Mexicans and see what they think. Or know.