No, not kidding, guess no one wants to play
A recent conversation with a gentleman at work showed me that some believe the term Silent Service was due to the fact that submarines move and attack silently beneath the sea, and attempt to avoid detection by sound.
The term silent service was used though because the men were not allowed to talk to the press, interviews were not granted as was common on surface ships, and there was a general 2 month blanket on submarine related news. They were silent in that they didn't speak to the press.
Perhaps this is universally known at Subsim however.