Youtube keeps track of all the videos you have watched. In some situations this feature can be quite useful, but on the other hand it may also cause some privacy issues.
To clear your Watch History, follow the below procedure:
Sign-in to your Youtube account.
In the left corner, click on "History"
Several options are available:
Selectively remove a video from the watch history: Check the box in front of the video > Remove.
Pause watch history - new videos won't be listed in the history, when the pause mode is enabled.