pack down
To press, push, or tamp something downward to compress it. A noun or pronoun can be used between "pack" and "down." I packed down the clothes as far as I could, but I still couldn't get the suitcase shut. We'll need to pack the soil down to create a strong foundation for the concrete.
pack something down
to make something more compact; to press something in a container down so it takes less space. The traffic packed down the snow. Pack the grass down in the basket so the basket will hold more.
pack down
[for something] to settle down in a container. The cereal has packed down in the box so that it seems that the box is only half full. Everything was packed down carefully inside.