Learn English – Where did the expression “falling down on the job” come from

According to Christine Ammer, The American Heritage Dictionary of Idioms (1994) the idea behind the phrase "fall down on the job" goes back to the nineteenth century:

fall down Fail to meet expectations; lag in performance. For example, It was disappointing to see him fall down on the job. This expression transfers a literal drop to a figurative one. {Second half of 1800s}

Not surprisingly, the earliest Google Books matches for the phrase occur in the context of work. The earliest of them are from the period 1904–1907. From Theodore Roberts, Hemming, the Adventurer (1904):

My Dear Captain Hemming: — Your stories reached me and were immediately set up and distributed broadcast. ... Be prepared to start East at the shortest notice, and please look up some one, an experienced man, of course, to keep an eye ob Cuba for us, should have to leave. A man who knows the country, and is immune from yellow fever, would be of more value than an experienced journalist. We have journalists here, but I fear they would fall down on the job.

In this instance "fall down on the job" unmistakably means "fail to perform adequately," but whether their probable failure—in the dispatching editor's opinion—is connected to the likelihood that they will fall ill in the tropical climate or to some other cause is not clear.

From The Wood-worker, volume 23 (1904):

We must have certain new machines, or certain new changes must be made before things will give the proper results. I find this more especially the case where the man has some scruples about being able to fill the position; he thinks that by making a whole lot of demands some of them will not be heeded, and if he falls down on the job this will be a means of letting him down easy—it was the fault of the machines, not the man.

From L.H. Robbins, "The Goddess of the Beach," originally for the Newark [New Jersey] News, reprinted in Book of the Royal Blue (October 1905), a publication of the Passenger Department of the Baltimore & Ohio Railroad:

Women have been men's encouragers since Adam got the sack from the garden. But they fall down on the job too often. The young man tries to tell his troubles to his best girl, and she, like Copperfield's Dora, doesn't want to hear about disagreeable things. The married man goes home from work with a burden of care upon his shoulders, and his wife wants to talk of the shortcomings of the washer-woman, the iceman or the grocer.

Here we already have not only a clearly metaphorical falling down on the job, but a clearly figurative job: acting as "men's encouragers."

And finally, from James Creamer, "Referendum Results," in Machinists' Monthly Journal (February 1907):

I also wish to to remind those would-be reformer that the duties of a Grand Lodge officer are somewhat different from the usual routine of a local lodge. It is all work and I am of the opinion that some who think it easy would soon fall down on the job or under it.