Web searches that you perform in the Yahoo! search engine become a part of your history. This means that when you begin typing in the Yahoo! search bar, your previous searches may appear as an option. "The record of search history which appears in the search box on Yahoo! is actually a feature of your web browser, and does not come from Yahoo!," according the Yahoo! website. You can remove the Yahoo! search history from the Safari "Preferences" menu.
Go to the "Safari" menu and select "Preferences."
Click on the "Autofill" tab. Click the "Edit" button next to "Other forms."
Click on "yahoo.com" to select it and press the "Remove" button. Press "Done."