So evidently the middle English had a word for Prussia, a variant of the then-Middle French Pruce (currently Prusse, if my high-school French hasn't mouldered away completely). This variant was Spruce.
Whence the name of the genus of tree (whose leaves provide an emergency source of Vitamin C), and (I am given to understand) the verb and adjective, after an apparently trim and tidy "Spruce leather." Those Germans were stereotyped for their meticulous cleanliness even then, it seems.
I'm seven thousand words in to August's transcribing binge, and brought my Czech-English dictionary home from work, where it had languished lo these several months.
I daresay I'm sprucing my writing up a bit.