Posterior

The posterior, individually buttocks, colloquially referred to as one's bum, butt, booty or arse, was the backside of humanoid life forms, and many other bipeds or quadrupeds. As well as depositing bodily wastes and gasses, the buttocks were also a common focal point of sexuality.

While talking about men with Gwyneth, Rose Tyler said she liked a "good smile" and a "nice bum". (TV: The Unquiet Dead) After psychografting herself into Rose's body, Cassandra O'Brien took great pleasure in examining her "nice rear bumper", slowly stroking her newfound posterior with one hand. (TV: New Earth) Toby Zed admired his "view" of Rose's backside whilst crawling through the air ducts of Sanctuary Base 6. (TV: The Satan Pit) Gwen Cooper complimented her boyfriend Rhys Williams on his "nice arse" while watching him get out of bed naked. (TV: End of Days) Minnie Hooper took the opportunity to squeeze the Tenth Doctor's bottom as he had a photograph taken with the Silver Cloak group, following it up by patting his backside. (TV: The End of Time) Miss Quill admired Coach Dawson's buttocks. (TV: The Coach with the Dragon Tattoo)

Alistair Gordon Lethbridge-Stewart was "not so bad [himself]" as posteriors went. (PROSE: Deadly Reunion)

Blon Fel-Fotch Passameer-Day Slitheen was "shaking [her] booty" while wearing Margaret Blaine's skin. (TV: Aliens of London)

The posterior could be augmented in a cosmetic surgery known as a buttock lift, which the Twelfth Doctor claimed to be utilised by humans obsessed with their image. (COMIC: Terrorformer)

According to Susan Foreman's diary, Francis Minto shoved a protractor into Gillian Roberts's buttocks in mid-March 1963, and she destroyed her desk in rage. Mr Grange strapped her three times across the palm of her hand for vandalising school property. (PROSE: Time and Relative)

When Victor Kennedy absorbed Bliss, her face ended up on his right buttock. When Elton Pope asked where she was, Kennedy lifted himself up, farting as Bliss told him he didn't want to know. (TV: Love & Monsters)

Undergoing parthenogenesis, Stacy Campbell found among the Adipose birthed from her, one emerged from her left buttock. (TV: Partners in Crime)

The phrase "kicking butt" was used in slang to describe besting an adversary. After successfully evading the surveillance of the Department with K9 Mark 2, Starkey announced that they had "kicked surveillance butt". In response, his robotic friend vowed to "kick surveillance posterior" whenever possible from then on. (TV: The Korven)

When the Twelfth Doctor was trapped within his confession dial and on the verge of despair, a hallucinatory image of Clara Oswald told him to "get up, off your arse, and win." (TV: Heaven Sent)

Bill Potts commented more than once on the Twelfth Doctor running like a penguin with its arse on fire. (TV: The Pilot, Smile)

Other references
Words relating to the posterior were also used as insults. The Testimony glass avatar version of Bill Potts angrily called the Twelfth Doctor a "stupid bloody arse" for not believing she was the real Bill. (TV: Twice Upon a Time)

John Hart commented that the Blessing looked like "a giant arse." (AUDIO: The Death of Captain Jack)