It may be that we would have to look for a paraphrase or a two-word expression, e.g. "great mouse". Just a vague idea for Latin: "mus + cretus" ( mouse + grown big). Perhaps one has not found more information about the origin because the idea is a word must be one word historically. That a lot of words were at first multi-word explanations that were shortened in the course of time and became a single word at the end is often not taken ininto consideration.