My phone ALWAYS autocorrects the F word to Duck. I am NEVER trying to use the word duck.
Someone on Instagram said they got around it by adding a new contact to their address book . . . and typing the F-word into the name field.
They gave it a first and last name . . . "[Effing] [Eff]" . . . so it would also stop auto-correcting to the word "ducking." So that's one way to do it. But there's another way that also works on most phones.
On an iPhone, just go to Settings . . . General . . . and find your Keyboard settings. Then click on "Text Replacement."
It's a feature that lets you create shortcuts. Like if you type, "omw" it'll change it to "On my way." But for this, enter the F-word as the shortcut AND the full phrase.
Android phones can do it too. Just go to Settings . . . Language & Input . . . and find a feature called "Personal Dictionary."
(If your iPhone has the newest version of iOS, you might not need to do anything. They apparently fixed it, but not all iPhonessupportthe newest version.)