The book, published in 1900, is about the way people in „high society“
and the „smart set“ around the turn of the century lived their lives. These
people sometimes adopted unusual mannerisms that formed part of their group
identity as the „smart set“, and they would play with language in ways which
people who were not part of „high society“ wouldn't understand.
There's an example of how these people „shortened (up)“ adjectives a few
paragraphs further down, where Miss La Touche describes the narrator's hat as
„too devey for words“. Here, „devey“ is a shortened form of
„divine“. This kind of language mannerism isn't used today and wouldn't be
understood. See also „Deevie and diskey“ here
(mleddy,blogspot).