n. A self-deprecating comment or story that acts as a brag because it shows the person is confident enough to admit a failing or embarrassment.
Other Forms
There are a lot of different kinds of brags: the humblebrag and the underbrag, for example. However, only one kind of bragging is not only acceptable, but encouraged: the shirtless brag.
—Sarah Anne Lloyd, “Which Star's Tour Workout Has Worked Wonders on His Physique?,” Wetpaint, December 20, 2013
The latest form of boasting is being called the 'underbrag' — whereby you tweet something so slovenly or shocking, it shows that you simply don't care or have to worry about losing your job/friends/partner.
—Claire Cohen, “Louise Mensch: hypocrite, self promoter and now fashion guru,” The Telegraph, February 13, 2013
2012 (earliest)
What's the underbrag? That's when you brag your own disaster or situation that one would not normally brag about. Skilled underbraggers can get away with doing so because the underbrag is not fake like the brag brag.
—Jen Doll, “Hello Underbrag, the Best Dang Bragging in the Whole Wide World,” The Wire, August 14, 2012