F*ck

F*ck was a profane word, sometimes used as a synonym for sex.

The word was used in the expressions "what the f*ck?" (TV: Everything Changes, Countrycide) and "f*ck knows", (TV: Random Shoes, End of Days) and the terms "f*cker" (TV: Countrycide) and "f*ck buddy". (TV: Out of Time)

Specific Usages
Ianto once screamed, "Show some f*cking mercy!" while Lisa was getting attacked by the Pterodactyl Myfanwy. (TV: Cyberwoman)

Gwen Cooper once shouted, "F*ck you!" at Jack Harkness after he taunted her for her affair with Owen Harper. (TV: End of Days)

Owen used the word to call a group of cannibals "sick f*ckers". (TV: Countrycide) He also used "F*ck knows" in response to a question about Eugene Jones. (TV: Random Shoes)

Rhys Williams once in a fit of rage told Gwen to, "sit the f*ck down!" (TV: Combat) He also used the word when, facing a hostile Nostrovite in the form of his mother, Brenda Williams, he exclaimed "F*ck!" when the chainsaw he wielded failed to work. (TV: Something Borrowed)

Upon discovering a bomb, Allen Shapiro gave his last words, "Oh, f*ck!", immediately before being caught in its explosion. (TV: The Blood Line)

Behind the Scenes
Before his casting as the Twelfth Doctor, Peter Capaldi was well known for portraying the character Malcolm Tucker in The Thick of It, who commonly used the word. This led to some fans joking that the Doctor would say f*ck at some point in the series.