The phrase “hit the deck” is an idiom that means to quickly drop down to the floor or ground in order to avoid danger or protect oneself. It is often used in military or nautical contexts to refer to a command given to soldiers or sailors to take cover or brace for impact.
For example, if someone hears an explosion nearby, they may shout “hit the deck!” as a warning for others to quickly drop to the ground to avoid injury or harm. In a naval context, the phrase may be used to instruct sailors to quickly take cover during an attack or to prepare for the impact of rough seas.
In a more informal context, “hit the deck” can also be used to refer to simply falling or collapsing onto the ground, usually as a result of exhaustion or shock.