It is a verb. It means literally to decorate with feathers or to congratulate yourself, i.e. feel self-satisfied. The uses of plume as a verb are very rare.

Hi Rich, “plume” in this context means to make oneself feel great satisfaction. Plume is normally associated with a bird’s feathers. A bird would tidy up its feathers to make itself look more attractive. You can also have a plume of smoke which starts off with a thin layer and then spreads into more layers, thus appearing as plumes of smoke going off in different directions just like feathers. I hope that helps you 🤓

Language changes over time. Little Women by Alcott was written about 150 years ago.