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.
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.