'Pound Cake' is so named because the original recipe consisted of one pound each of butter, sugar, flour and eggs. It originated sometime in the first half of the 18th century.
Here is a recipe from the 'Art of Cookery Made Plain and Easy' by Hanna Glasse (1747).
SOME CURIOSITIES IN HER RECIPE
Note the option of a pound of currants.
Note all the 'beating' - and all done with the hand - in particular the 1 HOUR time. That's one heck of a workout!
To Make a Pound Cake
Take a pound of butter, beat it in an earthen pan with your hand one way, till it is like a fine thick cream: then have ready twelve eggs, but half the whites; beat them well, and beat them up with the butter, a pound of flour beat in it, a pound of sugar, and a few carraways. Beat it all well together for an hour with your hand, or a great wooden spoon, butter a pan and put it in, and then bake it an hour in a quick oven. For a change you may put in a pound of currants, clean washed and picked.
Now compare the above with the recipe in 'American Cookery' by Amelia Simmons (1796) below:
One pound sugar, one pound butter, one pound flour, one pound or ten eggs, rose water one gill, spices to your taste; watch it well, it will bake in a slow oven in 15 minutes.