Johnny-one-note

Meanings

1. A person (or organization) who often expresses a strong opinion or viewpoint on a single subject or a few particular subjects.
2. An opinion or viewpoint on a single subject or a few particular subjects which is frequently expressed by a person or organization.

Etymology

Derives from the show tune Johnny One Note, from the Rodgers and Hart musical Babes in Arms (1937), in which the lyrics tell a story of a male vocalist named Johnny who sang with a band and could sing only one note, but sang that note extremely well.