I know what thermal underwear is for, but why and where did the name Long Johns come from?

Answer 1

The phrase “long johns”, used to describe long underwear, is often credited to John Stanfield of Nova Scotia-based company Stanfield’s, which has been in business since 1870.

While the design of the underwear is credited to Nova Scotia-native Frank Stanfield of Stanfield's Limited on December 7, 1915, the term "Long Johns" is credited to his brother, John Stanfield.

Answer 2

The earliest references to the garments strongly suggest the name came to wide public attention because it was applied to the long underwear issued to American soldiers during World War Two. Until this piece appeared, the first known reference to them was in a publication of 1943, but I’ve succeeded in taking that back a couple of years, to a letter home by a new recruit published in the Sheboygan Press, Wisconsin, on 16 October 1941: “We have had but three days of rain in the nine weeks we have been here. Last Friday it turned a little cool so we were issued our winter clothes. We all hope we don’t get our ‘long Johns’ for a while because it is too warm yet.”

Another local newspaper reference, this time from the Wisconsin Rapids Daily Tribune of 3 June 1944, suggests an origin: “Many a rookie has been ridiculed and laughed at the first time he swallowed his pride and donned his LONG JOHNS. They are the winter underwear issued by the Army, and have the disturbing effect of making a G.I. look like a scarecrow trapeze artist. It might be added that they itch but good! After a soldier finally gets into his LONG JOHNS, he invariably swells his chest, flexes his biceps and struts around the barracks like a John L. Sullivan, after whom these practical if not sightly garments have been named.”

Answer 3

Stanfield Long Underwear

Answer 4

This was a fun article:

Unmentionables Throughout History: The Naked Truth About Underwear
by Marjorie Dorfman
(link below)
"The history of Long Johns dates back to 1879 and begins in a most unlikely place: a boxing ring in Boston, Massachusetts. Big John Sullivan’s claim to fame was that he "could lick anybody, anytime, anywhere!" Modest though he may not have been, Big John became well known not only for his bare-knuckled style of fighting but also for his fighting apparel. His trademark was long underwear, aka the "Union Suit," which he wore every time he entered the ring. He became well known for both his fighting and his outfit, but his wardrobe went beyond even him, becoming the "long johns" the world has come to know and love (?) so well."