Learn English – The origin of the term half assed

By far the earliest instance of "half-assed" that a Google Books search turns up is from Thomas O'Brien & Oliver Diefendorf, General Orders of the [U.S.] War Department, Embracing the Years 1861, 1862 & 1863, volume 2 (1864), reporting on the court-martial of Captain John H. Behan on February 19, 1863:

Charge I.—"Conduct to the prejudice of good order and military discipline."

Specification 1st.—"In this ; that he, the said Captain John H. Behan, Company F, 16th Regiment Virginia Volunteers, while on duty in camp, on or about the 12th day of December, 1862, did use abusive and grossly insulting language to Joseph B. Hamilton, 2d Lieutenant of said Company F, before and in the presence of said Company F, while he, the said Joseph B. Hamilton, was on duty and was acting Adjutant of said 16th Regiment Virginia Volunteers, in words as follows, to wit: 'There goes our half-assed Adjutant ;' which was calculated to impair and weaken the influence and control of said Lieutenant Joseph B. Hamilton as Adjutant of said regiment, and also his influence and control over said Company. All this at or near Miner's Hill, Virginia, on or about the 12th day of December, 1862."

The captain was found Not Guilty on this charge, but Guilty on unrelated charges of having knowingly accepted a stolen sword from another soldier and of having refused to return to yet another soldier a sum of about $34 placed in his trust by that soldier.

The next instance of half-assed/halfassed that the search finds is from 1934, in Josephine Herbst, The Executioner Waits, where the closed-up form occurs at least twice. Here is one of them [snippet]:

He hardly listened to Jonathan until he caught the words, "And what I'm going to do is just light out, go to New York. I'm sick of these halfassed towns."

Numerous instances of half-assed turn up in search results from the late 1930s forward.

The first instance of half-arsed/halfarsed that the search turns up is from John Simon, Movies into Film: Film Criticism, 1967–1970 (1971) [series of snippets]:

The story, as everyone knows, concerns Myron Breckinridge, who, after a Scandinavian operation, becomes Myra. Equipped with physical beauty, knowledge from within of both sexes, and expertise in Hollywood lore, she sets out to capture both the men and the women of filmland today, and tomorrow the world. The movie turns all this into a dream, and cleans up the comically lewd incidents by reducing comedy to oafishness and lewdness to suggestive smirking. That kind of deliberately halfhearted — or, in this context, halfarsed — cleaning up is the real dishonesty, the real smuttiness of the enterprise. For the sexual acts are now performed by half-clothed Barbie dolls with carefully castrating camera angles, and if the dirtiness is not for real, it must be for dirty.

Interestingly, three useful collections from the period 1890–1915—Farmer & Henley, Slang and Its Analogues, Fla–Hyps (1893), Barre & Leland, Dictionary of Slang, Jargon & Cant, A–K (1897), and Thornton, An American Glossary, A–L (1912)—have no entry for either half-assed or half-arsed, despite the fact that the term clearly had been in use since at least 1862. Farmer & Henley is by no means squeamish about reporting on naughty words, so I find the term's absence there particularly baffling; I don't know how much the other two books may have been influenced by considerations of propriety.

Merriam Webster's Online, by the way, traces half-assed only to "circa 1932." Can the term really have gone underground for 70 years?