Idiot

Idiot was an insult used typically to suggest stupidity.

By the Doctor
The First Doctor called Ian Chesterton and Barbara Wright idiots for wanting to use the Dalek time machine to return home. (TV: The Chase) He later called Bret Vyon an idiot for killing Daxtar. (TV: The Daleks' Master Plan)

The Third Doctor called the Brigadier a "pompous, self-opinionated idiot." (TV: Inferno) He reaffirmed this when the Brigadier later authorised a transfer of the Nestene energy unit, and also held a low opinion of "that idiot" Brownrose. (TV: Terror of the Autons)

The Fourth Doctor said that computers were "very sophisticated idiots." (TV: Robot) He later called Harry Sullivan a "clumsy, ham-fisted idiot." (TV: The Ark in Space)

The Fifth Doctor recalled that his book series was to be named with a comma, The Doctor, Who Discovers but became Doctor Who Discovers after "some idiot" in the publisher's design department missed the "The", creating a logo that said Doctor Who. (AUDIO: The Kingmaker)

The Seventh Doctor branded Ramsay MacDonald, the Prime Minister of the United Kingdom in 1933, an idiot because his political programme concerned domestic economics and keeping the Conservative Party in check, rather than a focus on the rise of Adolf Hitler and the Nazi Party in Germany. (PROSE: Log 384)

Frustrated with the Tenth Doctor while unaware that he was a future incarnation, the Fifth Doctor branded him a "skinny idiot". (TV: Time Crash)

The Ninth Doctor repeatedly referred to Mickey Smith as "Mickey the idiot." Following his assistance against the Slitheen, however, the Doctor indicated that his sentiment had changed when Mickey observed that humans were idiots for not acknowledging the existence of alien life, adding that not all were. (TV: World War Three, Boom Town)

The Eleventh Doctor called Stepashin an idiot for attacking Skaldak. (TV: Cold War)

The Twelfth Doctor named himself "Doctor Idiot", having failed to realise that a forest had spontaneously grown across the Earth to protect the planet from a solar flare. (TV: In the Forest of the Night) Upon learning of incursion protocols to induct a President of Earth in the face of a Cyberman invasion, he bemoaned that voting for an idiot was the answer to everything, only to be told by Kate Stewart that he was the one and only candidate for the Presidency. He observed that 3W was a front for to exploit the wealth and the mortal remains of selected "idiots" to create a new race of Cybermen. He later called himself an idiot again when he figured out the answer to his question of whether or not he was a good man. (TV: Death in Heaven)

Though he used this term to describe himself, (TV: Death in Heaven) the Doctor later developed on this, seeing his own past error in his earlier claims of knowledge. The Twelfth Doctor believed that "only idiots" thought they knew the answers, and told Bill Potts he did not count himself among them. (TV: Into the Dalek) Though he previously wore this term with pride, he also advised Grant to "try not to be an idiot", "as much as it [was] possible for a human male." (TV: The Return of Doctor Mysterio)

The Thirteenth Doctor claimed that only idiots carried knives. (TV: The Woman Who Fell to Earth) She described Erik as "an idiot with a daughter who need[ed] him." (TV: It Takes You Away)

By others
Fabian openly wondered "what kind of idiot would steal a faulty TARDIS" as the First Doctor stole one from the repair shop where he worked. (TV: The Name of the Doctor) called the Ogrons idiots. (TV: Frontier in Space) Panna called the Fifth Doctor an idiot. (TV: Kinda) Rickston Slade called the Chief Steward an idiot, to which Astrid Peth objected. (TV: Voyage of the Damned) Donna Noble called Klineman Halpen an idiot for not realising why the Ood did not resist being enslaved. (TV: Planet of the Ood) As the worldwide nuclear grid prepared to launch nuclear missiles against a Sontaran flagship, which would have easily withstood such weaponry, General Staal observed with admiration that "the bravery of idiots is bravery nonetheless." (TV: The Poison Sky) Leef Apple Glyn Slitheen-Blathereen branded her husband Tree Lorn Acre an idiot for letting Sarah Jane Smith escape. (TV: The Gift) At Kerb!am, Graham O'Brien observed that the System was "still an idiot boss" despite being automated. (TV: Kerblam!)

Behind the scenes
The Idiot's Lantern was named for a disparaging term for television.