The Long Tale of a Shirttail
For such an unassuming word—it’s not even a garment, just a part of one, and the bottom bit, at that—shirttail has undergone a surprising amount of semantic development. This fact is clearly reflected in the treatment it has received in successive editions, supplements, and versions of the Oxford English Dictionary (OED), from 1914 to the present day, as more and more of the story has been uncovered.
In 1914, the printed instalment or fascicle of the original New English Dictionary (NED)—which was to become the OED in later incarnations—covering the alphabetical range shastri to shyster was published. It’s evident the original editors didn’t feel that shirttail (or rather, the hyphenated shirt-tail) justified much houseroom. It appears in a list of ‘Simple attributive’ uses (i.e., transparent uses modifying another word) of the main entry shirt, with no definition, and is illustrated by a single quotation from C. G. Leland’s 1873 work The Egyptian Sketch-book:
Rushing madly about, their blue-and-white shirt-tails waving in the wind.
Presuming this was the only piece of evidence for the word that the editors had to hand, one can see why they thought it was deserving of this straightforward and cursory treatment.
By 1933 more illustrative evidence had become available to the dictionary’s editors for a vast array of words, and among this evidence was another morsel for shirt-tail, which was included in the entry containing additional material for the word shirt in the Supplement to the Oxford English Dictionary published that year:

What’s notable here is what a bizarre-looking example this is. ‘Made a shirt-tail across the prairie’? What on earth is supposed to be going on there? That’s one large piece of linen. Yet the 1933 Supplement didn’t seem to think it was worth elucidating further, carried away perhaps with the excitement of having found an earlier example.
Fast forward to 1986, and a second supplement to the OED was published in four volumes. A great deal more evidence had become available in the interim, and the humble shirt-tail was finally deemed worthy of a definition—two, in fact—and much-expanded treatment:

This is how the entry (or, technically, subentry—it was still housed under the main entry for shirt) stayed in the second full edition of the OED (OED2) in 1989, and how it appeared until the present update of OED Online.
It’s striking here that an incredible amount of information and extra evidence has been added, but in a very condensed and codified format, partly due to the space constraints of publishing a work as vast as the OED in print.
Such constraints are not applicable to online publication, and it is in this context that I enter the story of shirttail. Tasked with revising it for OED Online, several things about OED2’s treatment struck me immediately. Firstly, as far as chronology goes, it seems extremely unlikely that the very earliest use available should be for the specific, extended usage shirt-tail boy. Secondly, although the definition for sense (a) is straightforward enough, sense (b) seems to be a concatenation of a number of different things; ‘something small or insignificant, or a remote relationship’ does not seem to represent a particularly logical pairing. Thirdly and finally, it seems still that no-one has seen fit to explain how one might make a shirt-tail across a prairie.
The first point was addressed quickly. Extensive research, carried out by the Research group which forms part of OUP’s Oxford Languages department, is a standard element of revision of material for OED Online, and in this case they found gold in the form of usage of the main, standard sense of shirttail almost 200 years earlier than less internet-enabled versions of the OED had managed, from 1659 and in quite an intriguing context:
One of the Schollars hung a piece of his shirt-tayl or linnen, or some such thing, upon the Gown of the Mayor’s Officer, in contempt and derision to the Mayor.
Take that, Mayor’s Officer! And, by extension, the Mayor! Message very much received, I feel sure.
The second point was a case of teasing out the various strands of meaning involved. This involved the shirttail finally cutting its ties with its parent garment, as a word if not as a physical item. The variety of meanings displayed by OED2’s erstwhile sense (b) alone suggested that more expansive treatment was, for a third time, required.
The resulting online main entry for shirttail shows three distinct adjectival uses, all specific to the United States, describing: a very young child (especially a boy), something small or insignificant, and a distant relation.
In all cases our research uncovered earlier uses, and also strongly indicated that the ‘small, insignificant’ strand is historically mostly represented by disparaging references to small businesses. It also became clear that although OED2 implies that the ‘shirttail boy’ sense arose from the more general ‘small’ sense, the opposite is true: for 40 years people referred to very young children in this way before the use was transferred to businesses, countries, and other things regarded as small or ‘immature’. The reason for the original application is unclear: perhaps it relates to young boys being renowned for not tucking their shirts in, or perhaps to the fact that in rural American summers it was once customary to dress very young children in a long shirt and nothing else; but it’s impossible to say for sure.
Addressing the third point, the shirttail you can make across a prairie, embodied another benefit of the historical textual resources that the internet affords us. In trying to establish what on earth was going on, I discovered that the 1933 Supplement had in fact mistranscribed the quotations from the original book, William Drummond Stewart’s Altowan, or, Incidents of Life and Adventure in the Rocky Mountains, edited by one J. Watson Webb:

Ah, OK, so it’s a straight shirt-tail. One can begin to see what’s going on—a shirttail streaking out behind someone running flat-out—and further research demonstrated that this was a well, if briefly and no longer, established American phrase, with supporting evidence found from 1839 until 1914.
Coincidentally, 1914 brings us back full circle to NED first registering—as something of an afterthought, by the looks of it—the modest (as it seemed then) existence of a word for the lower part of a shirt. Since then it has gained a definition, more illustrative quotations, main-entry status, even more definitions, even more illustrative quotations, and, finally, lost a hyphen: this latter decision based on analysis of OUP’s bespoke corpus of modern English usage, where we found that the single-word form is more common than both the hyphenated and two-word (i.e. shirt tail) forms put together.
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