Back in December 2008, I came up with an "interesting" solution to iOS's annoying habit of "correcting" the word that I wanted with the word that it thought I wanted. The problem was colloquially known as the "Ducking iPhone" for reasons you are probably smart enough to figure out on your own. (If you're confused, here's the PG–13 version, or, for the more sensitive types, TUAW also has a PG-rated version.)
The solution was simple: Create a new contact in your iOS address book, using the word you want iOS to learn as the first or last name. The good news is iOS is much better about learning words (tap the 'x' which appears when the auto-correct option appears), but a Twitter search of 'ducking iPhone' shows that many people are still running into this problem.
If you would rather not add contacts to your address book, there is another option now which did not exist in 2008. On your iOS device, go to Settings » General » Keyboard » Shortcuts. Tap the + on the top right to create a new shortcut, and then enter the word that you want iOS to learn in the "Phrase" field. You can leave the "Shortcut" field blank. Tap 'Save' and enjoy your freedom to use whichever words you want.
However, having done (admittedly limited) tests using both systems of adding words to iOS, I still think the Contacts solution is the better one. I have noticed that when words are entered into my iOS Contacts list, iOS will use them as autocorrect suggestions when I start typing. Sometimes iOS will even suggest autocorrections based on "names" from the Contacts list, but does not seem to do that for the Shortcuts. Siri and iOS' dictation will also use your Contacts for matching spoken words.
So if you have a favorite word that iOS doesn't recognize, whether it's one of the 7 words you can't say on television or some obscure idiom of your own choosing, you can keep iOS out of your ducking way.