I Haven't "Shown It" To Anyone Recently, Either

I don’t know the back of my hand all that well

I believe canonically, you're supposed to be pretty familiar with it. 

I know!  That's what makes this so troubling for me.  I guess I know a couple of random facts about the back of my hand that nobody else would ostensibly be aware of, but that's about it.  I frequently get a small patch of rough skin on the second knuckle of the middle finger on my right hand and I have no feeling in the back of my left index finger (left over from air-bag chemical burns in 1996).  That said, now you all know about those as well, and they're more facts about the backs of my fingers than the back of my hand, so I don't know if that even counts. 

Truth be told, I don't sit around staring at and studying the back of my hand all that often.  This seems especially weird considering I spend a considerable amount of time working on computers (in both my actual job and my little hobby of writing hopefully comedic nonsense on a Blag), so my hands spend a great deal of time in more or less my direct sight line.  You would think this lends itself to a greater familiarity and understanding of the back of my hand, but you would apparently be wrong.  

I bring this up for two reasons.  One, the phrase "know this like the back of my hand" came up in an audio book recently, and my brain swirled down into its usual deep spiral of banality.  Second, I have this bruise on the back of my left hand, and I have no earthly clue how I got it.