In the searchable copies of this work that are online, this word does not occur in the letter from Phillips or anywhere else in the book.
However, whenever you see this word in writing from before the Civil War, it probably means the breaking up of the United States. This is, of course, what started the Civil War. Many people in the South wanted disunion because the North was opposed to the spread of slavery. Some people, in the North, like William Lloyd Garrison wanted disunion with the South because they did not want to be united with slaveholders.
So the word simply means the breaking up of the Union of the USA.
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