Some secrets are destined for the grave.
To speak or not to speak? That was actually not the original question here, though the word mundtot - literally, "mouth dead" - might sound like it. In Old High German, munt meant protection. So in legal terms, someone who was munttot was without protection and couldn't defend themselves.
Later, people started thinking that the word must have something to do with mouths, so the spelling and the meaning evolved just slightly. Now someone who is mundtot is not in a position to make a public statement. Those with bad intentions can make someone else mundtot by blackmailing or threatening them. The victim doesn't necessarily die, as the term might imply - but had best take their secret to the grave.