That feeling you get when you wonder how you ever survived without something/someone and have difficulty remembering your life before it/them.
The name refers to P.G Wodehouse's characters Reginald Jeeves and Bertie Wooster. Jeeves was Bertie's butler and soon, Bertie became so dependant on Jeeves that he wondered how he got along without him.
My dad: I use the internet so much these days, I wonder how we kept in touch before it was invented.
Me: Yeah dad, it's a real Jeeves Phenomenon.