Posterior

The posterior, (TV: Regeneration) individually buttocks, (TV: The Coach with the Dragon Tattoo) colloquially referred to as one's backside, (TV: Rosa) bum, (TV: Death of the Doctor) butt, (TV: The Korven) booty, (TV: Aliens of London) arse (TV: End of Days) or ass, (TV: Combat) was the rear end 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)

Owen Harper complained that he was "freezing [his] arse off" as Torchwood Three tended to the corpse of John Tucker. (TV: Everything Changes)

Arriving to investigate an unusual death at Nightspot with Torchwood Three, Gwen Cooper told PC Andy Davidson that there was no point in him "freezing [his] arse off out here". (TV: Day One)

Following a Weevil attack, Jack Harkness left a message on Owen's voicemail telling him to get his "boney little ass" to the Hub. (TV: Combat)

When a sleeper agent disguised as a paramedic stopped the oil tanker he was driving in the middle of a road, one car driver angrily ordered him to get back in the truck before he "shove[d] it up [his] arse." (TV: Sleeper)

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)

Allen Shapiro repeatedly called Brian Friedkin a "fatass". (TV: End of the Road)

Following Jorjie Turner from 2050 to November 1963, Starkey confered with K9 Mark 2 to "grab her and get [their] butts back through" the portal of the Space-Time Manipulator, to which K9 concured "rapid transport of butts, imperative." (TV: The Cambridge Spy)

Owen Harper told Toshiko Sato "sometimes I think even that stick up your arse has got a stick up its arse." (TV: Greeks Bearing Gifts)

Owen observed that Maggie Hopley was a "pain in the arse". (TV: A Day in the Death)

Andy Davidson once told Gwen Cooper that she had "a face like a slapped arse." (TV: Adrift)

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

Ace called Gilgamesh "a right royal pain in the arse." (PROSE: Timewyrm: Genesys)

Ram Singh called Tom Dawson an "arsehole". (TV: The Coach with the Dragon Tattoo)

Behind the scenes
A behind the scenes photograph from the production of TV: Aliens of London/World War Three, released on the BBC website, offered a view of the Raxacoricofallapatorian posterior, specifically that of the Slitheen, with the humourous caption, "Does my bum look big in this?"