One would think a smarty-pants smartphone like the iPhone would only have to be told a new non-standard word once to remember it forever and ever. And, actually, you'd be right but it is the method of inputing the new words that is the key here.
Using some apps like Notes to enter non-standard text will not work. For whatever reason, Apple has deigned to not allow some applications to update the dictionary database.
So how do you do it? Open up Safari and enter the words into a Google search and whammo, they are instantly added to your dynamic dictionary.
Seems simple enough but I don't get why you can't update this dictionary from any application much less any native Apple app?
Oh well, now we can all happily add all the delicious swear words we want and the auto-correct will no longer try to replace "fucking" with "ducking". And life will be oh so much better because we can swear more easily. Oh boy!