In Norse mythology, the Yggdrasil was a massive tree connecting the nine worlds of the universe. The name is commonly accepted to be from yggr, meaning "terrible", and drasill, meaning "horse". The reason for this is very complicated. Yggr here was actually in reference to the god Odin, who was nicknamed "The Terrible", among many other things. Drasill metaphorically meant "gallows", because there was an Old Norse expression about gallows being the "horse of the hanged". So Yggdrasil was "Odin's gallows", because in one legend he hanged himself from the tree for nine days and nine nights so he could understand the secrets of runes. Finally, I couldn't find anything on the derivation of yggr, but drasill is thought to come from the Proto-Indo-European root der, meaning "to support".
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.