I'll admit that, even as a word guy, the presence of "scadutree" confused me. But later it became clear, and many online sources confirmed, that much like the old English names for the other two trees, Erdtree (Earth Tree) and Haligtree (Holy Tree), scadu is derived from Old English for shadow. The original spelling was sceadu and would have been pronounced something like "sheadow", which of course evolved into the modern English shadow.
Like the Evergaols being from Old English for Ever Jail, gaol being pronounced much the same as the modern word. And various other examples throughout the game that I can't recall.