Who originally defined leverage scores to be the diagonal elements of $X(X^TX)^{-1}X^T$? A nice description of leverage in the sense that I am using it is given here so I will not repeat it.
Who originally defined leverage scores to be the diagonal elements of $X(X^TX)^{-1}X^T$?

Hoaglin & Welsch (1978) say
Thus we use the hat matrix to identify "high-leverage points." If this notion is to be really useful, we must make it more precise.
This suggests the term 'leverage' is not widely used at that point, and I don't have an earlier reference.
Tukey (who also coined the term 'hat matrix') uses 'leverage' in a related sense in a 1965 paper, talking about the amount of information in subsets of a sample, but not for regression. It's not used in the paper by Cook defining Cook's distance for measuring influence.
