Long johns is most probably an American term of surprisingly recent origin, and differs slightly from the terms union suit and combination suit. We will examine the definitions of long johns, union suit and combination suit, where these terms came from and some examples of their use in sentences.
Long johns is another term for long underwear, which is long-legged and long-sleeved underwear. Long johns are usually but not always two pieces. The term long johns was first used during World War II to describe the long underwear issued to American soldiers. The inspiration for the term long johns is uncertain, though some believe it refers to the boxing champion John L. Sullivan. Sullivan lived and boxed around the turn of the twentieth century, much earlier than the first known use of the term long johns. However, he was still well known during World War II and wore a costume similar to long johns during his boxing exhibitions. Note that the term long johns is singular and plural, and that the proper name John is not capitalized in the term.
A union suit is long underwear that is all of one piece. The American term is union suit, the British term is combination suit. The union suit was created in America in the mid-1800s and was originally designed for women. The term union suit comes from the fact that the top and bottom of the long underwear were united, and the term combination suit comes from the fact that the top and bottom were combined.
Examples
On New Year’s Eve while most people were ringing in the start of 2017, Bob (who for privacy reasons asked his last name not be used) walked outside his house in just his long johns, slippers and shirt to get firewood for his fireplace. (The Petoskey News-Review)
If you’ve ever heard of the union suit, you’d probably refer to it as “the onesie with the flap in the back.” (The Huffington Post)
While the combination suit later became more prominent due to its convenience, what was most significant when it came to undergarments wasn’t what you wore but what it was made of. (The St. Albert Gazette)