If something is many-sided, it has a lot of different qualities or features — not one simple face. A many-sided person has many skills or moods; a many-sided problem has many causes and no single easy answer. It is a slightly careful, written kind of word; in speech people often say “it has many sides” instead. You will see it in school writing about people, books, and arguments. The picture behind it is a shape with many sides, like a box. Though actually we almost always use it about character and ideas, not about geometry class — if you mean a shape with many sides, people usually say polygon, or just name the shape.