"To stop ones mouth, that he cannot tell out his tale."
Blount's Glossographia, Thomas Blount, 1656
From Latin obacerare, meaning 'to contradict'; obacerate means to contradict, to disagree, and to shut the other person up. Some dictionaries claim that it means 'to shut someone up before they can finish speaking.' Others leave it at 'to shut someone's mouth'. But really, they can say whatever they want, because this is one of those words that appear to be used only in dictionaries.
Obacerate and its sister obaceration do not show any evidence of actually being used by anyone. The 17th century lexicographer Thomas Blount collected words willy-nilly to create his dictionary, and some of his findings are of dubious origin. He probably did not actually invent the word obacerate out of whole cloth, but we have not yet found where it might have come from. His dictionary collected words from many languages, along with words used in the street, and it is quite possible that this was a one-off usage by someone who thought that it sounded clever to use a Latin word in English form. Whatever the original source, it has been passed down from dictionary to dictionary for centuries, and has been completely ignored by the rest of the world.
Well, that's not quite true. There are a lot of people who like odd words, and many have found its meaning relevant to their lives and works. There are some who use it as a user name; it has gained some slight popularity in Harry Potter fanfic as a magical spell; and there are in fact at least two bands named Obacerate. It took 350 years, but people are starting to use this word -- of course, it is nowhere near the point where it could be justifiably added to any dictionary, but give it another couple of decades and it might just get there.