Gin and Jag is a phrase used to describe upper middle classes. The sort of people who drive Jaguar cars and drink gin and tonics. Also the gin and jag belt is an area where these people are common. It can be shortened to JJ's.
"How was last night?"
"It was alright. The party was in the Gin and Jag belt though"
"So was it full of JJ's?"
"Not as may as I thought"