How well do you actually know the back of your hand?

If you were to ask me, I’d say it depends on which hand you’re referring to. I know the backside of my right hand well. There’s a quarter-inch scar just below my ring finger. And my pinky knuckle is scarred from days in college I would bare-knuckle punch my heavyweight punching bag. 

But my left hand… can’t tell you much about it.

It’s an idiom with unclear origins. But the earliest appearance is in John Collis Snaith's 1916 novel The Sailor. He uses it twice in the book to make his point: “So much had he knocked about the world that he knew men and cities like the back of his hand” and later on he writes, “his native city of Blackhampton, certain parts of which he knew like the back of his hand.”

When we consider our hands, we’re either breathing into them during these cold months to keep them warm or staring at our palms before we wash them. But to tell me you know it like the back of your hand will leave me questioning your level of familiarity with the topic.

But according to the Oxford English Dictionary, it means “to be thoroughly familiar or conversant with.”

If you actually think about it, to know something like the back of your hand – a part of your body you rarely look at – fails in its purpose.