"tie down" means to tie so as to prevent something from lifting up or leaving a floor or flat surface. For example, you would tie down the cargo on a ship's deck, or a load on the back of a truck. org/wiki/tie_down * Though you could also tie down those things if specifically securing them flat to a surface.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.