While many of these words have, more or less, become either archaic or severely specialised over time, several—fourteen of which are listed below—are still going strong in 2014.
Beneath each word, I have included a Google Ngram. These show the relative frequency of words taken from a corpora of around 5 million books published since 1800. In most cases, you will see that the words on this list did indeed fall into usage during the First World War. However, one or two words existed prior to the outbreak, but with entirely different meanings. In these cases, I was able to trace the origin of the present day meaning—with a great deal of help from the Online Etymology Dictionary—back to World War I.
There are certain words—such as Blighty—that are often cited as having originated during World War I, but that in fact either a) pre-dated it or b) contained conflicting reports as to their etymology. In the name of accuracy, I opted to leave such words off the list.
Note: while every single word on this list was coined during World War I, it is possible that one or two did not fall into usage because of World War I. In other words, a couple of entries, while originating between the years 1914 and 1918, may have been coined in light of non-war-related circumstances (this is unverified, either way). The first entry on this list, however, is not one of those words.
In its current form, the word blimp is first attested from 1916, but the actual origin of the word—meaning a small, rigid airship—is unknown. One theory suggests it might have originated from the term "Type B-limp", the opposite of "Type A-rigid."
The chiefly American English phrase bullshit (the noun form) fell into usage in or around 1914. Interestingly, the Ngram below shows written usages appearing only as early as the 1940s. However, due to censorship, the omission of swear words from written texts was fairly typical throughout the first half of the twentieth century.
While this word clearly existed prior to World War I (as indicated by the Ngram below) the meaning "dug-out fortification" is believed to have originated at some point during the Great War. Regardless, we can see from the Ngram that usage of the word increased dramatically in British English after 1914.
Prior to 1917, there was no such word that exactly described the notion of "blending into one's natural surroundings to hide oneself from the enemy." The word camouflage, as attested in Popular Science Monthly (August, 1917), changed all that.
Interestingly enough, this chiefly American English term—used in reference to a fictional childhood disease (much like lurgi in British English)—was coined by British soldiers in 1917 to mean "body lice." Previously, however, the word enjoyed limited usage as a nautical reference.
In the sense we understand it today, the word cushy—meaning "undemanding, easy, or secure"—is an Anglo-Indian word and is derived from the Hindi phrase khush, meaning "pleasant, healthy, happy." It is first attested in this sense from 1915.
In the sense of "aerial combat", this word is first attested from the First World War, though like bunker, it enjoyed non-military usage in the decades prior. The military usage, however, didn't really take off (pun absolutely intended) until World War II (see Ngram below).
Once again, this word had existed prior to World War I (meaning that a person had been accidentally gassed). However, the military definition "gassed in a military attack" is first attested from 1915.
9. Nose-dive (verb)
While the noun form of the word—as it pertains to airplanes—was coined just before the war (1912), its verb equivalent (as in, "the plane started to nose-dive") is first attested from 1915.
Amazingly, the current definition of posh ("the quality or state of being elegant, stylish, or upper-class") did not exist until 1914. Previously, it had been used to mean "money" and more specifically a "coin of small value, a halfpenny."
11. Pushing up the daisies
With so many fatalities during World War I, it was always inevitable that a euphemism for "dead" would propel itself into the language. Per The American Florist, the phrase pushing up the daisies is first attested from 1917.
12. Shell shock
While the term shell shock has developed a more figurative definition in recent years, it was nonetheless another product of 1915, where it was used to describe "psychological disturbance caused by prolonged exposure to active warfare."
Once more, this word naturally saw earlier usage through other definitions, but the meaning "armored, gun-mounted vehicle moving on continuous articulated tracks" was first attested in 1915, as army tanks became used in wartime for the very first time.
14. Trench coat
Now known more commonly as a high street fashion item, a trench coat had previously been worn only by British troops during World War I and, as such, is first attested from 1916.
Are there any other words still in use today that were coined during World War I? Let us know in the comments below.