underbum

under- +‎ bum, by analogy with underboob.

underbum (plural underbums)

1. (Britain, slang) The lower part of the buttocks, as revealed by certain clothing such as skimpy shorts.
   • 2014, Jess Cartner-Morley, "The rise of the underbum: how to flash the flesh this summer", The Guardian, 5 July 2014:

     Now, no one is pretending this is an easy look to pull off, but whereas the whale tail was shaming because it was a fail – the byproduct of you scrabbling in the bottom of your bag for your Oyster card – the underbum is at least bold and brazen, in a lollipoppy, Katy Perry or Rihanna kind of a way.

   • 2015, Catherine Shoard, "The strange case of Fast & Furious star Paul Walker", The Guardian, 14 April 2015:

     They’ve always been a big hit at the box office: ethnically diverse, incredibly loyal, keen wearers of white vests (for the men) and underbum-showcasing hotpants (for the ladies).

   • 2015, Chelsea White, "Cheeky display! Kesha flashes some underbum in a pair of very short denim cut-offs as she heads to lunch", Daily Mail, 5 November 2015:

     Not done with just showing underbum, the front of the star's shorts featured distressed holes which the top her thighs showed through.

   • For more examples of usage of this term, see Citations:underbum.

• underbutt (North America)