1
general::
Speak in public, especially at great length. For example, Barbara loved to hold forth on the latest discoveries in astronomy. [Second half of 1600s]
American Heritage Idioms
2
general::
Phrase(s): hold forth (on someone or something)
to speak at great length about someone or something. • Sadie held forth on the virtues of home cooking. • Sharon is holding forth, and everyone is paying close attention.
McGrawhill's American Idioms And Phrasal Verbs