There are many reasons for wanting to hide your viewing history. Some are legit, others are not so pure of heart, so here is how to do it.
Log into your YouTube account.
Click the little arrow in the top right corner near your username.
Click on Video Manager from the drop-down list
Go to the History section on the sidebar.
Click on Pause All Viewing History. Now from this point on, none of the videos you watch will be documented.
Image from Google