Office collects words that you frequently use in the current session. When you later type the first three letters of a collected word, Office automatically completes the word.
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.
Choose Tools - AutoCorrect - AutoCorrect Options - Word Completion.
Unmark Enable word completion.