(idiomatic) Having only one opinion, outlook, tone, etc., especially as expressed repetitively; without variety or range.
Example: 1971, Michael Sragow, "Theatre: Look Back in Anger Tonight at the Loeb Ex," Harvard Crimson, 13 March (retrieved July 25, 2009):
But Pope Brock plays him in such a one-note key of gulping and spitting and snickering cynicism that the spectacle becomes numbing.
1992, Jane Creighton, "Bierce, Fuentes, and the Critique of Reading," South Central Review, vol. 9, no. 2, p. 66:
The footnotes that attend Ambrose Bierce in the U.S. literary canon roughly place him as a minor writer of grotesque supernatural tales and trenchant war stories, a misanthrope, curmudgeon, a purveyor of stringing sarcasms, a one-note wit.
2005, Anahid Kassabian, "Academic Frostbite (A Cautionary Tale)," Women's Studies Quarterly, vol. 33, no. 3/4, p. 403:
To his mind, there was only one right and true position on the question. This sort of one-note response is precisely the problem facing politically engaged academics in the U.S. at the moment.
2009, Mary Pols, "Year One: Jokes from the Stone Age," Time, 18 June:
The movie is one long snigger. . . . It might be one-note, but at least it's in the key of funny.