When you write messages, e-mails or notes on the iPhone, the built-in dictionary helps you by suggesting the spelling of the words you want to type. It also corrects seemingly incorrect terms. Sometimes it happens to spell a word correctly, only the dictionary does not recognize it. The iPhone will then suggest replacement words, or it may replace it with a similar word. To avoid this, add it to your mobile dictionary. At that point the device will no longer give you suggestions when you write it.
Steps
Step 1. Tap the gear icon on the iPhone home screen
The device settings menu will open.
Step 2. Tap the "General" option
The general settings menu will open.
Step 3. Scroll slightly through the general settings menu until you find the entry "Keyboard"
At this point, tap it to open the settings menu.
Once the keyboard configuration is open, check that the following items are disabled: AutoCorrect, Spell Check and "." Shortcut
Step 4. Tap "Add Shortcut" or "Text Replacement", depending on the iOS version of your iPhone
Step 5. Tap the "+" or "Edit" sign to add a new word
Step 6. Type the word you want to add in the "Phrase" box
You can leave the "Shortcut" box blank. Once done, tap "Save".
Step 7. Finished
You will be able to see and use the entered word without the automatic check.