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?