Untidy means messy or not neatly arranged. An untidy bedroom has clothes scattered on the floor, books piled haphazardly on the desk, and a bed that hasn't been made. An untidy backpack is stuffed with crumpled papers, loose pencils, and yesterday's lunch wrapper.

The word describes both spaces and appearance. Someone with untidy hair might have it sticking up in different directions. An untidy worksheet has eraser marks, cross-outs, and writing that wanders across the page.

Untidy sits between “messy” and “disorganized.” A room can be untidy without being truly dirty: maybe everything's clean, just not put away. Scientists often keep untidy desks covered with papers and specimens because they're focused on their work, not organization. But an untidy locker makes it hard to find what you need when the bell rings.

The opposite is tidy, meaning neat and well-organized.