The word “plumose” comes from the Latin: “pluma” meaning down or “plumosus” meaning full of down or feathers. Further exploration suggests that it means something like “having multiple filaments coming off one axis, like a feather”. This gets more interesting when you look at the historical examples: “a plumose leaf”, “plumose tentacles”, or as in this example “plumose anemone”.

Basically, “plumose” means “feathery but not, you know, having anything to do with actual feathers”