Origin of the word "Merchant Navy"

It was originated by King George V following their service in the First World War.

Merchant seafarer's primary job was to carry food grains, edible oil and cereals to the Royal Navy soldiers during the world war. They were not issued with any kind of arms and ammunition, as their primary job was to look after the Royal navy soldiers food supply only, not to fight the battle.

This Merchant Navy (MN) identification badge was issued to seaman on board British ships in January 1940.

The Merchant Navy name have been adopted in Britain in 1922, as part of King George V's formal acknowledgement of merchant seafarer's service and sacrifice during the First World War. That conflict cost the British Empire's merchant marine - as it was then better known - 2500 ships and 15000 seafarers.

During the First World War, merchant seafarers were not issued with any sort of identification badge, and as they did not wear uniforms many were made feel uncomfortable ashore. The issue of MN badges during the second World War was designed to indicate that the wearer was taking an active part in the war effort along with the Royal Navy.

Gundecking

In the modern Navy, falsifying reports, records and the like is often referred to as "gundecking." The origin of the term is somewhat obscure, but at the risk of gundecking, here are two plausible explanations for its modern usage. The deck below the upper deck on British sailing ships-of-war was called the gundeck although it carried no guns. This false deck may have been constructed to deceive enemies as to the amount of armament carried, thus the gundeck was a falsification. 

A more plausible explanation may stem from shortcuts taken by early Midshipmen when doing their navigation lessons. Each Mid was supposed to take sun lines at noon and star sights at night and then go below to the gundeck, work out their calculations and show them to the navigator. Certain of these young men, however, had a special formula for getting the correct answers. They would note the noon or last position on the quarter-deck traverse board and determine the approximate current position by dead reckoning plotting. Armed with this information, they proceeded to the gundeck to "gundeck" their navigation homework by simply working backwards from the dead reckoning position.