by Richard Whitlock, a medical doctor. Probably used even then with a taint of impractical idealism. Modern pejorative use seems to have begun on the socialist left, mocking those who were unwilling to take a hard line. OED has this citation, from "The Nation" in 1923:
"There is nothing the matter with the United States except ... the parlor socialists, up-lifters, and do-goods."
The form do-gooder appears in Amer.Eng. from 1927, presumably because do-good was no longer felt as sufficiently noun-like. A slightly older word for this was goo-goo