Robert Altman's 1970 Korean War comedy M*A*S*H is acknowledged as the first American mainstream movie to use the word "f*ck."
It was uttered by actor John Schuck, and in 2021 on the podcast "Gilbert Godttfried's Amazing Colossal Podcast!," Schuck explained that it was ad-libbed.
It occurred during a football scene, in which the some of the actors were actual NFL football players, including Chicago Bear Ben Davidson. Schuck said:
"I had to line up opposite Ben Davidson, and Andy [the second unit director] said, "Say something that will make him angry." So, I just said, "Alright, bub, this time your fucking head's coming right off." And that's the last thing I remember for five minutes.
"He cold-cocked me. I mean, he just knocked me backwards. I saw stars and birds and tweety-birds, the whole thing. And they gave me some smelling salts, and I came around, and there he was, all 6'10" of this guy: 'I'm sorry, man, but you can't talk about my mother that way, blah blah'
"So, the next day we go to the dailies, and that scene comes up, and Altman liked it so much, bless his heart, he kept it in.
Watch movie history being made at 4:07, if you're not faint of heart.
We've come a long way since then, and in today's enlightened age, children don't even need the accompaniment of an adult to hear one or two"f*cks" in a movie. (But three is right out.)