Definition of prunes-and-prism in English:

prunes-and-prism

(also prunes-and-prisms)

adjective

  • Designating a prim or affected facial expression or manner of speaking. In extended use: affected mannerisms, superficial accomplishments.

Origin

Late 19th century; earliest use found in The British Weekly: a journal of social and Christian progress. A phrase notionally or actually spoken aloud in order to form the lips into an attractive shape.