Think of the word sunder as violently tearing something apart. A frequent line in a wedding ceremony is, "What God has joined together, let no man tear asunder." Keep that in mind, and you'll have the meaning of the word.
Throughout its history, the word sunder has carried the same basic meaning, "to tear or break apart." It is of good Germanic stock, coming from the Old English verb sundrian. All of the related Germanic words, then and now, carry the idea of separation, usually in a violent or unpleasant manner.