No matter where the name long johns really comes from, what's for certain is that men have worn similar articles of clothing since at least the 19th century. From the mid-1840s, in fact, men wore something called "union suits," a type of clothing like long johns and an undershirt made from similar material, but all in one piece, according to Merriam-Webster — a long john onesie, if you will. Union suits, as Merriam-Webster also writes, served a dual purpose: to keep the person wearing them warm, but to also protect outer clothing from sweat.
Today, the term long johns can mean pants-length underwear with a so-called "trap door" in the back and a separate undershirt, like a union suit, just in two pieces. For others, though, long johns — sometimes called thermals or simply long underwear — refer only to the ankle-length underpants, as Merriam-Webster elsewhere notes. As can be seen, union suits and long johns are clearly related in both form and function. It's under that latter definition, though, that the most common and prevalent theory of where long johns come from has its roots.
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