the “f” word is never used by anyone or anything in Halo. It is however used by players stuck in the grav lift room of the T&R on legendary…
I can vouch for that second part, for sure.
Curse words, and Halo
Though it will keep coming up, Marty O’Donnell, Halo’s Audio Lead, makes a very clear statement about the existence of a certain curse word in Halo: