The American-English mild insult your mother wears army boots—and variants using your sister, army shoes and combat boots—seem to have originated amongst teenagers and young adults in the second half of the 1940s.
This insult perhaps originally alluded to impecuniousness. I have found the following in The New Orleans Item (New Orleans, Louisiana) of Monday 13th December 1920:
“Hiking Nellie Cruise” Woman Hobo, Reaches N.O.
En route from St. Louis to Mobile, “Hikin’ Nellie Cruise,” America’s best known woman hobo, applied for and received a night’s lodging at the Seventh precinct police station Sunday night. “Hikin’ Nell” is 47 years old, wears army shoes, and has been a member in good standing of the Amalgamated and Protective Brother (and Sister-) hood of the Knights and Ladies of the Road—in other words, a hobo—for the past 19 years.
I have found two occurrences of an earlier similar insult, to wear cotton drawers, which was directed to men, in newspapers published in New Mexico:
1-: From The Carlsbad Current (Carlsbad, New Mexico) of Friday 5th November 1909:
Impressions of a Tender-Foot
By Will Robinson
About Curry’s Resignation *
It is a little hard to see where the optimist can find anything to cling to in the resignation of Gov. Curry, the news of which was given to the people of New Mexico last week. I am a natural born optimist. Whoever says to the contrary is a liar and wears cotton drawers.
* The U.S. military officer and politician George Curry (1861-1947) served as the Governor of New Mexico Territory from 1907 to 1910 (his resignation, announced in November 1909, only took effect in March 1910).
2-: From The Philosophy of Defeat, published in the Santa Fe New Mexican (Santa Fe, New Mexico) of Tuesday 14th November 1911:
I traveled some thousand miles and delivered a couple of dozens speeches that were not rotten except in spots.
The man who says I said a mean, an untrue or an unchivalrous thing in any of them is a liar and wears cotton drawers.
These are the earliest occurrences that I have found of your mother wears army boots and variants—presented in chronological order:
1-: From the column From The Reporter’s Notebook, by Joe Rathbun, published in The Times Recorder (Zanesville, Ohio) of Thursday 25th September 1947:
Something new in insults has been added. A couple of local youngsters were on the outs the other day and insults were being exchanged at a rapid pace. Finally the littlest came up with this clincher: “Aw your mother wears army shoes!”
2-: From Stirring Pitt and Pen State Rallies Held, published in the Pittsburgh Post-Gazette (Pittsburgh, Pennsylvania) of Saturday 22nd November 1947—the grammatical subject is Penn State, i.e., Pennsylvania State College:
Students and old grads of Pennsylvania State College and The University of Pittsburgh held pre-game rallies in Downtown Pittsburgh and Oakland.
Observers at both rallies agreed that the night belonged to the sign-painters. Placards hung from the necks or lapels of old grads, students and well-wishers carrying messages of defiance.
“Plaster Pitt” found rebuttal from Pitt students in “Stew State” and an ambiguous “Penn State Wears Army Shoes.”
3-: From the column Pitching Horseshoes, by Billy Rose (William Samuel Rosenberg – 1899-1966), published in many U.S. and Canadian newspapers in January 1948, for example in the Santa Rosa Republican (Santa Rosa, California) of Saturday the 3rd—one Major Timothy Morgan tells the following story to the author:
“Some years back,” the Major began, “I was shopping for ten crates of celery at the Washington Market. […] Two baritones were feuding at the Metropolitan Opera House. One had enlisted my services to put the squitch on his rivals [sic] performance at the opening of ‘Don Giovanni.’ I had assembled a claque of juvenile delinquents and it was my plan to have them chew enthusiastically on stalks of celery during the baritone’s more emotional passages.
“While strolling through the produce market, I passed some chicken crates. Suddenly I heard a voice say, ‘Yer mudder wears army shoes.’ The voice seemed to be coming from a crate. I peered in. A Plymouth Rock looked me straight in the eye and cracked, ‘Yer mudder takes dope.’”
4-: From Gazette Team Rushes to Interview Spring Robin But Find Little Harbinger in Untalkative Mood, by Brian Cahill, published in The Gazette (Montreal, Quebec) of Tuesday 9th March 1948:
We hasten to report herewith the appearance of the first robin of spring. […]
A team of Gazette staffmen have been for the past few weeks haunting city parks, and other open places in the municipality in a determined effort to scoop the nation on the story. One of them spotted the little fellow yesterday […].
Our man whipped out pencil and notebook, and requested the interview usually given on such occasions by the cheery harbinger of sunny spring.
“Drop dead,” said the cheery harbinger of sunny spring.
“Aw, yer mudder wears army shoes,” said the robin.
5-: From Press Pass No Open Sesame, Cartoonist’s Sad Discovery After Embarrassing Heave-ho From Movie, Wrestling Match, by Wilbur Arkison, published in The Gazette (Montreal, Quebec) of Saturday 1st May 1948:
We are going to give the youth a lecture on the virtues of respect for his elders and the proper way to address the senior members of the staff but we remembered what happened the last time we tried that. We were very patient and father-and son-ish but when fe [misprint for ‘we’] finished all the comment we got was something which sounded like “Aw your mother wears army boots.”
6-: From the account of the second annual Kiwanis Pushmobile Derby, published in the Pottstown Mercury (Pottstown, Pennsylvania) of Thursday 29th July 1948:
Before the race the waiting competitors were like any other group of contestants before the event—tense. . nervous, vacant-eyed, open-mouthed.
After the race it was a different story—as usual.
One conversation between two rivals, obviously still on the best of terms, was, “Hey, yer father wears sneakers to church.” To which the reply was, “Ahhh, ya mudder wears Army shoes.”
– meaning and origin of ‘Maggie’s drawers’
– notes on ‘all fur coats and no knickers’
– meaning and origin of the phrase ‘to get one’s knickers in a twist’
– ‘Charlie’s dead’ (your petticoat is showing)
– meaning and origin of the phrase ‘to go commando’
– a crude phrase: ‘to see a woman’s breakfast’