The grue- in gruesome is a now-extinct Middle English verb meaning "to shudder", while -some is just the familiar suffix used to denote the existence of a quality. Grue was first recorded in a fourteenth-century Scottish dialect as grew, and both words were relatively rare until the late nineteenth century, when novelist Sir Walter Scott popularized the adjective through his writings but didn't really use the verb much. Due to its history and cognates in languages like Dutch, some etymologists think that grew could be of Scandinavian origin, while others contend that it all comes from Middle German or Middle Dutch grewen, both of which would be from the Proto-Germanic reconstruction gruwijana, with the same definition. Gruesome is unrelated to gruelling, which comes from another Germanic root meaning "grain".
Hello! I'm Adam Aleksic, a sophomore studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy politics, vexillology, geography, board games, conlanging, and law.