Green’s Dictionary of Slang

(US) of a man, to wear no underpants.

2008		Simon & Burns ‘Get Some’ Generation Kill ep. 1 [TV script] You wearing shorts? It’s going to be hot, man. Shit, I’m freeballing all the way to Baghdad.
2012		www.jobuddy.blogspot.com 29 Apr. 🌐 What kind of underwear do you wear? Boxers, or freeballing... [Simes:DLSS].