The idiom hold forth implies talking for a long time about a subject that interests the speaker but not necessarily the audience.

Hold forth: to talk at great length; harangue

 

When we left, he was still holding forth on World War II.

 

(Dictionary.com)

I first came across this phrase in Stephen King's novel The Tommyknockers, where it's used to describe a character drunkenly ranting about nuclear power long after everyone else at the party wished he'd stop.

The idiom hold forth implies talking for a long time about a subject that interests the speaker but not necessarily the audience.

Hold forth: to talk at great length; harangue

When we left, he was still holding forth on World War II.

(Dictionary.com)

I first came across this phrase in Stephen King's novel The Tommyknockers, where it's used to describe a character drunkenly ranting about nuclear power long after everyone else at the party wished he'd stop.