This word means the complete degree of penetration and you seem to double the "through" to make it sound stronger. I wonder, in Russian you say "along and across" to mean that you know something "inside and out", you know something very well, deep into the subject/matter. I wonder, have you the same meaning in English, the first "through" means "throughout vertically" and "throughout horizontally", resulting "through and through" -- both directions. I know the subject throughout, no matter which side we are looking at it. This is how I look at this phrase. Am I right or there is another origin?
I think you are taking it too literally. Whether you mean it literally or metaphorically, it just means "completely through", not "through in both directions". For example,
I don't think that means the boar was literally run through both along and across, or vertically and horizontally.
To amuse himself till his lordship was at leisure, he took down a volume of Shakspeare splendidly bound, and on opening it discovered, from the gilding, that it had never been read ; also, that the worms were eating it through and through. Some dozen years afterwards, another visitor took down the same volume, and found the following lines pencilled by Burns on the first page : — " Through and through the inspired leaves
Ye maggots make your windings ;
Again, that seems to mean "completely or repeatedly through", not "through in both directions".
I would say the equivalent English phrase is "backwards and forwards."
We also like to say "I know something like the back of my hand."
I've also been wondering about this phrase "through and through" and finding a disappointing lack of info on its history. The only mention of original use I found was to mean a sword piercing armor and flesh - I'm guessing "through the armor and through the flesh" got shortened to "through and through," which became a figurative phrase meaning "thoroughly" in any sense.
The way it means in every direction is in uses like
"He's through and through an evil man"
"It was filled through and through with chocolate cream."
It means completely. I think his basic concept is a good one since through by itself is a one-dimensional thing. It's usually directional. Through and through tends to indicate two or three dimensions. If a boar was stabbed through and through with spears, it would indicate many of them.