Your iPhone compares the words that you type against a built-in keyboard dictionary to spell-check, auto-correct, predict and suggest alternate words as you write. Initially, this one-size-fits-all dictionary lacks the special words, slang terms, shortcuts or acronyms that you use in your personal and business communications, but your iPhone learns with repetition. Adding your custom terms to the dictionary dynamically as you type, it eventually recognizes them as legitimate words after several repetitions.
Type a note or message that includes your unusual acronym, technical term, shortcut or slang word. You can even include a non-English word or term that derives from a combination of two languages, such as "Spanglish."
Tap the "X" of the replacement word that iPhone suggests beneath your strange or curious word. This dismisses its suggestion and coaxes your iPhone into adding your unusual term to your dictionary.
Repeat Step 3 two or more times until your iPhone eventually recognizes your uncommon term as a legitimate word. You'll know that a custom word has been successfully added to your dictionary when your iPhone quits trying to auto-correct it or begins to suggest it as a legitimate word.