All right-thinking people, when they see the word darcy, think immediately of Mr Fitzwilliam Darcy of Pemberley in Derbyshire.
Illustration by Hugh Thomson (1860 - 1920).
(He's nothing to do with this post, I'm just mentioning him because he's such a cheering person to think about, even though he was never the man for me. For one thing, I don't think he'd look kindly on my current jeans-and-a-jumper outfit (though I do probably play the piano as well as Elizabeth Bennett).
A darcy is a unit of the permeability coefficient of rock. Its symbol is D. A rock has a permeability of one darcy if it lets through one cubic centimetre of one-Poise-thick fluid in a pressure gradient of one atmosphere per centimetre acting across an area of one square centimetre.
(Or something like that, anyway.)
Entertainingly, you can also have a millidarcy.
Presumably a millidarcy is a thousandth of a darcy; but I'm going to pretend that's the name of Elizabeth and Fitzwilliam's slightly scatty daughter.
Word To Use Today: darcy. The darcy unit is named after Henri-Philibert-Gaspard Darcy (1803-58). The Poise unit was named after Jean Léonard Marie Poiseuille.