Thickly means in a way that is dense, heavy, or closely packed together. When snow falls thickly, fat flakes come down fast and pile up quickly. When you spread peanut butter thickly on toast, you use a generous, heavy layer instead of just a thin smear.

The word describes how densely something is arranged or applied. A forest might be thickly wooded, meaning the trees grow so close together that little sunlight reaches the ground. Fog might roll in thickly, making it hard to see more than a few feet ahead. Someone with a strong accent might speak thickly, with their words shaped heavily by the sounds of their native language.

Thickly can also describe how something is done with fullness or substance. If you slice bread thickly, you cut big, substantial pieces rather than thin ones. The word suggests abundance and density: when paint is applied thickly, you can see brushstrokes raised on the canvas. When grass grows thickly, it forms a lush, full carpet rather than sparse, patchy growth.