In the Comic Strip,

Union Suits were the underwear worn by the entire cast of characters, well the menfolk for sure. 

You've already met a couple already, the boy who lost his britches in the Sadie Hawkins Day Race and Pappy Yokum (see my previous two postings). There's Li'l Abner, of course:

There were others who traipsed around day and night in their long, one-piece underwear too. 

For example, Black Rufe:  

and Hannibal Hoops who wasn't shy about running around in his union suit either:

Three fellahs who lived in Dogpatch, USA 

sometimes chose to wear Boxer Shorts over

long-handled, button-down underwear!

Who? Well, the Yokums! Especially when they were asked 

to do an ad for Fruit of the Loom Underwear!