Most of the examples I gave before of humor from the trenches were of the mild, safe, Jay Leno style of humor, a style we associate with the larfs of the past. But gallows humor has always burbled beneath the surface of our culture, and has had its periodic fads — the “sick humor” boom of the late 1950s or the dead baby jokes of the 1980s are examples from long before Rick and Morty.
WWI happened to fall right in the middle of a run of Little Willies and Ruthless Rhymes. (This site is worth perusing in serious detail for its rich collection of, and information about, such matters.) So it would be strange if there existed no grim jokes from the war. Yet even the A.P. Herbert poems, while sometimes dark, were still, like Punch, respectable. Certainly the irony of “We’re Here Because We’re Here” is morbid, but it’s not Little Willie morbid.
The phrase “funny as a crutch,” glossed helpfully by the free dictionary “as not funny at all,” is in fact a relic of Great War-era gallows humor. According to this book, which I also highly recommend, the original meaning of “funny as a crutch,” from the days during and right after the war, when discharged soldiers limped home, was “really, really funny.” It was morbid, but it was also morbid humor.
This 1917 Dictionary of Similes lists “funny as a crutch” right after “funny as a barrel of monkeys” and “funny as a clown.” This 1919 magazine starts a page of humorous verses with the make-of-this-what-you-will dedication:
To my dear Self I dedicate this page:
In modern mien I hide myself so much:
Though I am really funny as a crutch,
And should receive a very golden wage.
Other period citations are more ambiguous or clearly mean “unfunny.” 1910 is the earliest example I could find, which obviously predates the war, but it is so layered in snark that I can’t tell, when it calls Ibsen “funny as a crutch,” whether it means Ibsen is so maudlin that it’s hilariously kitsch or that nothing about it is funny at all.
The sense sometimes seems to be “very funny, but only in a horrible and grotesque way,” and I guess that makes sense.