Green’s Dictionary of Slang

prudish, puritanical.

1926		Golden Book Mag. 4 576: [The] Nice Nelly literature of our forties and fifties; you don't get many of the [...] raw , uncensored impulses of the human stream-of- consciousness.
1934		S. Walker City Editor 160: [S]ometimes the best phrase—indeed, the only precise phrase—is slangy and reeks of the streets. If so, don’t spoil it by Nice Nellie quotation marks.
1943		S.M. Kootz New Frontiers in Amer. Painting 55: His technique is strong, and contemptuous of nice-nelly standards.
1952		G. Dixon in Daily Mirror (N.Y.) 8 July 4/5: I’ve got to use ‘heck’ [for hell] because practically all my editors are ... being more rabidly nice Nellie than usual.
1969		J.R. Kidd Education for Perspective 128: We are safe, dull, gray and stodgy. I believe the term that best describes this is Nice Nelly.
1978		G. Vidal Kalki 193: You make the Mafia seem nice-nelly.
1999		North Shore News 23 Aug. 🌐 She wrote repeatedly to our do-good, nice-Nelly (to everyone but law-abiding, middle-class Canadians) government.