AppLocker can password protect individual apps on your Mac. It's easy to use, and there is absolutely no configuration required. Just start AppLocker, add a password, and select the apps you want to keep private.
Use this tool and stop worrying about your privacy when you borrow your computer to a guest, friend, or family member. No one will mess with your important apps anymore.
Note: While the software is classified as free, it offers in-App Purchases.