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1.
Underoos is undergarments containing pictures, graphics of cartoons character(s) and/or movie character(s) on a t-shirt sold with a color brief. These eventually evolved into briefs only and two major underwear manufactures took the concept and branded FunPals/FunGals (Fruit of the Loom) and Showtoons (Hanes). These underwear have the cartoons and/or character(s) printed directly on the brief in either multiple impressions or large single impression. However, these designed were mostly limited to children under the age of 12 with some design prints such as Star Wars with sizes available for slightly older children. In response to popular demand of kids not being able to wear this underwear manufactures such as Old Navy and Gap now market printed briefs to older children. Following the movie Charlie’s Angel a prototype adult version of the original design of underoos was created by Fruit of the Loom and was introduced to their product line, however they were unable to secure a distribution partner to continue into full scale production. Hanes is currently marketing such underwear as Space designs, Ninja Turtles, Hulk and Spiderman designed to older children and seem to be continuing their efforts with some success as they are continuing to introduce different designs.

Underoos in general urban format refer to any underwear other then solid color (that is, red, blue, tightie whities and so forth), including boxer briefs and in some cases boxers that contain cartoon or movie character(s). Since the majority of boxers contain cartoon characters they generally do not get the title underoos since cartoon boxers are not in the minority category of underwear such as briefs.
Should I wear underoos to school today or should I wear my tightie whities?
by Frank Williams April 17, 2007
 
2.
Superhero themed brief style children's underpants (e.g. superman, spiderman, batman, the hulk, etc.) Very popular during the '80s. When used in reference to someone older than approximately 12 implies immature and/or freakish qualities of said person.
Jehannum kicks wedge in the underoos
by Misuta Supakulo March 24, 2004
 
3.
A silly name for underpants, specifically "tightie-whities".
"Hey, can I come in?"
"No! I'm in my underoos!"
by D45 June 12, 2006