Citations:underbum

Noun: "(UK, 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.
 * 2016, Sasha Brady, "We need to discuss the impracticalities of Kylie Jenner's ripped jeans (and why Irish women will never wear them)", Irish Examiner, 31 August 2016:
 * "The denim hotpants with the underbum cleavage was a big look this summer but with the jeans you're getting a little more coverage.
 * 2018, Catherine Keane, "Fashion must haves for Festival Season", Connect (Country Kerry), August 2018, page 18:
 * One of the more unfortunate fashion trends in my opinion of late has been the 'underbum' - ie, baring faintly obscene amounts of your rear end with festival-goers been [sic] the worst offenders.