LibreOffice collects words that you frequently use in the current session. When you later type the first three letters of a collected word, LibreOffice automatically completes the word. If you want, you can also save the current list of collected words so that it can be used in the next session.
If there is more than one word in the AutoCorrect memory that matches the three letters that you type, press Ctrl+Tab to cycle through the available words. To cycle in the opposite direction, press Ctrl+Shift+Tab.
By default, you accept the word completion by pressing the Enter key.
To reject the word completion, continue typing with any other key.
Choose Tools - AutoCorrect - Word Completion .
Uncheck Enable word completion.