noun. Nastywear worn in the nether-region. Or on your head, if you're a Thong Bandit.
Real citation: "That's a sad loss to the music industry. He was a giant (both literally and figuratively) in the business and anybody who can get women to throw their undernasties on stage at them will forever be a hero of mine."
(July 7, 2003, British Expatriate Network, http://www.british-expats.com/archive/index.php/thread-6993.html)
Made-up citation: "I'm a cool customer. I never get my undernasties in a bunch. I never get my panties in a wad. Only once--in the eighties, during a bad week that involved an astronaut and a duck--have I ever gotten my speedo in a twist."