It is modern
The only reference I have seen regarding it is "Truman Library."
There are zero references contemporary to Lee that he ever said those words.
As a matter of fact, if one looks at the
google ngram viewer (click search on this link), this word appears twice in the 19th century, in 1897 and 1898, and there is a nice peak in the 1940s, Truman's era, and then in the '60s...