I have a distinct recollection of being very young (I think seven) and first hearing the phrase to know something like the back of your hand. It was in a children's story and somebody was saying that he knew this forest like the back of his hand, or somesuch. It worried me greatly as I didn't really know the back of my hand at all. I would, to this day, have trouble picking my own hands out of a police line-up. I know this forest like my own home would be a far more illustrative expression.
I never really recovered, but I do feel an awful lot better now that I know that the back of my hand is the opisthenar. I don't really care whether I know something, so long as I know the word for it. Opis is the same root as opposite, and thenar is Greek for the palm of the hand.
Incidentally, palm trees are named after the palm of your hand because of the shape of their leaves.
So though you may not know the back of your hand (barring tattoos), you can now, at least, give opisthenaric compliments.
I also remember a children's book that had the line "Every Englishman knows in his blood that the moon rises fifty minutes later each day." This caused me no end of consternation as I had no idea about this lunar retardation and was forced to consider the terrible possibility that I might be secretly French.
Might be mine, really couldn't say.