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A particular point in a conversation with someone where you find someone has ceased to regard anything you say and are quite content to have a continuous monologue with themselves. Any attempt by you to re-join the conversation is usually ignored.

A term used frequently around the South of England amongst school children and college students. Nobody is completely sure of its origin, or in fact, if it is inspired by a true person.
Jerry: Hey Tom, how long has he been talking?
Tom: Last time i checked, about half an hour.
Jerry: Do you think we should maybe ask him what the hell he's going on about?
Tom: I wouldn't bother, he won't listen, he's having a Dinsdale Moment.

Barny: Alright John, what's your opinion on the Green party policies?
Greg: Shit Barny! what are you doing? You actually want John to have a Dinsdale Moment?
Barny: Sure, it's hot in here and we can use his mouth as a fan.
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