Underoos often is used to refer to briefs with cartoon characters on them.
Fruit of the Loom created the Underoos brand in the 1970s, which had t-shirt and underpants sets and differed from tighty-whities. At the time, it was only marketed to children. In the 2010s there were new versions of underoos for adults. FTL also created Funpals and Fungals character briefs. Hanes had the Showtoons brand. Those three brands are not sold anymore.