A crashing bore
Definition 1 of 1
Pro Tip 1/3Strong Disinterest
Emphasizes that the subject is so boring it's almost impressive. 
The lecture was such a crashing bore, I almost admired the speaker's dedication to dullness.
Pro Tip 2/3Informal Use
Used in casual conversation, not in formal writing or speeches. 
My date turned out to be a crashing bore with endless stories about traffic.
Pro Tip 3/3Slight Rudeness
Can be seen as offensive if used directly about someone in their presence. 
Be careful not to call someone a crashing bore in a social setting; it's quite rude.