How come sand means courage/pluck?
There isn't much information available on the Internet regarding its etymology.
With word etymologies I think the buck stops with the redoubtable World Wide Words, but in this case it doesn't sound thoroughly convincing:
Sand here has just the same sense as the older grit, clear grit, or true grit, that refer to a person who has strength of character, pluck, stamina, the ability to see things through to the end. The reference here, presumably is to the toughness of grit, especially that in gritstone, a common name for the material that made up the stones of a corn mill.
Why sand should suddenly pop up in its place — sometime near the end of the 1860s — is hard to say, though it is an obvious enough synonym.
Mark Twain uses both grit and sand in successive sentences in that place in Huckleberry Finn: “She had the grit to pray for Judus if she took the notion — there warn’t no backdown to her, I judge. You may say what you want to, but in my opinion she had more sand in her than any girl I ever see; in my opinion she was just full of sand”. How these became interchangeable terms is a small mystery that needs to be resolved, but don’t bank on anyone finding the solution anytime soon!
[World Wide Words]
Anyone in the know of the roots of sand as used in this sense?