One of the most common fear I have seen in my friends is that they often ask me, if some one knows or can get to know which videos you have watched earlier on youtube with their google account connected and logged in, as these days you would need to connect a gmail or google account with youtube account and google account is linked to other services as well like orkut, gmail, google reader & calendar etc.
However after good amount of search, I came to know by default only your youtube friends would know which videos you might have watched. Many people on google forums have been requesting google and youtube team to add a new option to which could allow them to disable youtube video viewing history, as people have added in caps that “PLEASE, ADD THE "DISABLE VIDEO HISTORY" OPTION IN YOUTUBE ACCOUNTS” you can check out this google forum thread here
Now as there is no option as if now in youtube which could stop them recording your youtube video viewing history, and you might have a feeling of insecurity that any one would get to know which videos you have watched on youtube, however I have seen that people could get to know which youtube channel you have subscribed, to know which channels you are subscribed go to this link.
No most the explicit videos on youtube ask you to login with your youtube account, so there is no way you can watch these videos without login to your youtube account so the final option to make sure that no one knows about which videos you watched, is that manually you go, login with your youtube account and clear your youtube videos watched history, follow the procedure below.
1. Go to youtube.com and login with your google or youtube account credentials.
2. Once you are logged in, go to the youtube account settings as shown in the image below.
3. Now go to section My Videos & Playlist by clicking the option in the top left corner of the page on youtube account settings as shown in the image below.
4. Now select the history option in the left pane to see all the videos you have watched on youtube.
5. Under the history section, you will see the links to all the videos you have previously watched till now with their thumbnail and at the top you will see the button to clear, remove or delete youtube video watched history, so that no one could know what videos you saw on youtube as shown in the image below.
You will need to confirm your action as well once again as shown in the image below.