The word spruce was first borrowed into Middle English in 1378 with the spelling sprwys. Back then, it meant "of or pertaining to Prussia", because the tree was traditionally associated with the country. That's an alteration of Prus, the Old French name for for Prussia, but nobody is completely certain why the s got added the the beginning of the word. The only theory I see out there is that it got conflated with spruse, a generic term for commodities traded by Hanseatic merchants that also existed around the time. It's no coincidence that the word Prussia sounds like Russia; both it and Prus come from the Proto-Slavic root Po-Rus, meaning "before the Rus" people who went on to found Russia. Beyond that, there are no Indo-European reconstructions; seems like everything about this word is shrouded in intriguing obscurity.
Hello! I'm Adam Aleksic, a sophomore studying linguistics and government at Harvard University, where I founded the Harvard Undergraduate Linguistics Society. I also have disturbing interests in politics, vexillology, geography, board games, conlanging, and law.