Hopkins is famous for making up words that (a) sound like what they mean, and (b) sound like they really ought to be real words. There are some great examples of this in "Spring and Fall." He makes up the word "unleaving" to describe leaves falling off trees. It's like "undressing," but for trees. And then he makes up the word "wanwood" to describe dead leaves. "Wan" means "pale and sickly," so again—this makes sense.
Added bonus: "wanwood" is alliterative! And the word "leafmeal" describes the messy, disorganized heap of leaves on the ground—like the word "piecemeal," only for leaves.
A big part of Hopkins's poetic project was to make us look at everyday objects—like dead leaves—in new ways. Making up new words to describe things helps his readers to look at them with fresh eyes. Cool, right? Challenge: make up one word per day for a week, and try to get your friends to catch on and start using them! You'll be the coolest kid at your next poetry reading!