Definition of "boxers"

Across the street was the well-adorned window of an underwear store. In amongst the displays of briefs, boxers and boxer-briefs, Forrest detected a square-jawed man staring straight back at him.

2011, Damon Norko, The Delilah Factor, page 157

But right now I'm waiting at the bus stop, waiting to go to work, bundled in two jackets and a coat, briefs under boxers under sweat pants under jeans.

2015, Max Reagan, Thoughts of Ink, page 46