The origin is uncertain and therefore unknown. However, there is a theory that the garmnent was named after the heavyweight boxer John Sullivan who used to wear a similar garment in the boxing ring. The same company has been making this garment for 225 years
Thermal Underwear
long johns
long johns Thermals union suit
Val Johns's birth name is Valjean Johns.
Daniel Johns's birth name is Daniel Paul Johns.
Margo Johns's birth name is Jessie Margaret Johns.
Cort Johns's birth name is Cort Maclean Johns II.
Hugh Johns's birth name is Hugh Richard Lewis Johns.
Stratford Johns's birth name is Alan Edgar Stratford-Johns.
Cathy Johns's birth name is Cathy Campion.
According to Michael Quinion[2], "John" may be a reference to boxer John L. Sullivan, who wore a similar looking garment in the ring. This explanation, however, is uncertain and the word's origin is ultimately unknown.
People should not laugh at if a person wears Long Johns. Most of the time Long Johns cannot be seen they are wore under the pants.