Wiktionary
a. 1 (context nautical English) Of a ship so far distant that only the masts and sails are visible above the horizon 2 (context military English) Of a tank, stopped with its hull protected by ground to the front, so that its occupants or weapons have a line of sight forward. alt. 1 (context nautical English) Of a ship so far distant that only the masts and sails are visible above the horizon 2 (context military English) Of a tank, stopped with its hull protected by ground to the front, so that its occupants or weapons have a line of sight forward.
Wikipedia
In sailing and warfare, hull-down means that the upper part of a vessel or vehicle is visible, but the main, lower body ( hull) is not; the term hull-up means that all of the body is visible. The terms originated with sailing and naval warfare in which the curvature of the earth causes an approaching vessel to be first visible "sails-up." Beginning in the 20th century, hull-down has also been used in armoured warfare.
In modern armoured warfare, hull-down is a position taken up by an armoured fighting vehicle (AFV) so that its hull (the main part of the vehicle) is behind a crest or other raised ground, but its turret (or a superstructure or roof-mounted weapon) is exposed. Turret-down is the position in which the vehicle's crew can observe forward from roof hatches, but the vehicle is completely hidden (usually a few metres further back from a hull-down position). The belly armour should not be exposed, because it is vulnerable to even modest antitank weapons.
Usage examples of "hull-down".
The busiest people were the engineers, who were digging holes for all the tanks so that they could fight hull-down, with only their turrets showing.
Two of my Hellbore bolts strike home, while the third expends much of its megaton fury on a ruin-strewn ridge sheltering one of the targets as the enemy Bolo went hull-down behind it.