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. When Martha requested that she swear just once, Chantho complied by saying "no". (TV: Utopia)
Behind the scenes Edit
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)
The BBC Wales series sometimes teases at swearing, cutting it close in editing for humour. One 2017 episode, Thin Ice, cheekily cuts away as Bill utters: "No sh—". Back in 2006, New Earth cuts from Cassandra calling Rose Tyler "that little—" to Rose saying, "bit rich".