Swearing

Swearing was the use of speech considered rude.

The Ninth Doctor once mentioned that the TARDIS' translation circuit had a swear filter that prevented the passengers from hearing any swear words; even an angry cavewoman's rants were translated as her saying, "Blinking", rather than a more apt phrase. (PROSE: Only Human)

The Malmooth Chantho respectively began and ended her sentences with "chan" and "tho"; as she explained to Martha Jones, to not do so would be akin to swearing. (TV: Utopia)

Yasmin Kahn's grandfather often swore loudly in Punjabi when he accidentally injured himself. He hoped that his young granddaughter hadn't understood, but she had. (AUDIO: Fallout)

The Twelfth Doctor's internalised anger sometimes lead to a preponderance of swear words appearing on the psychic paper. (TV: Dark Water)

Behind the scenes
The Ninth Doctor's explanation about the TARDUS translation systems is a reference to the lack of swearing in televised Doctor Who, which has remained a family programme since its inception. Indeed, William Hartnell, who played the First Doctor, specified that the show had "no sex or swearing" in a 1964 interview. (REF: The First Doctor Handbook)

One 2017 episode, Thin Ice, cheekily cuts away as Bill utters: "No sh—".