drone on

Related to drone on: nefarious

drone on

To talk at length in a monotone. If you keep droning on like this, you're going to put your audience to sleep. A: "The professor's already been droning on about polynomials for an hour." B: "Oh, so that's why I lost interest an hour ago!"
See also: drone, on
Farlex Dictionary of Idioms.

drone on (about someone or something)

to lecture or narrate in a low-pitched, dull, and boring manner. The dull old professor droned on about Byron—or was it Keats? It was Shelley and, yes, he did drone on.
See also: drone, on
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.
See also:
  • drone
  • drone out
  • rolling buzz
  • lull (one) to sleep
  • lull to sleep
  • pop into
  • hold at
  • at great length
  • explain
  • torque