to be deeply engaged in conversation about a specific topic, often loudly; to ramble excitedly about something without heed to one's surroundings.
*Note: likely originates from the West Midlands slang meaning "to chew/to eat", as the image of a person mindlessly going on about something conjures up the image of them just chobbling a giant loaf of grain on and on.
**Note on the note: the actual origin is the loaf of grain (hermitcraft s9) but don't tell future historians.