underbum

plural underbums

1. UKslangThe lower part of the buttocks, as revealed by certain clothing such as skimpy shorts.

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

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

   2 more examples

   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.

   2014 July 5, Jess Cartner-Morley, “The rise of the underbum: how to flash the flesh this summer”, in The Guardian

   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 April 14, Catherine Shoard, “The strange case of Fast & Furious star Paul Walker”, in The Guardian

   Synonyms: underbutt