Mary marries John. Now Mary's brother Mario and John are Schwäger (singular, Schwager), brothers-in-law. John's sister Johanna and Mario, on the other hand, are Schwippschwager (male) and Schwippschwäger (female), respectively. That also means that Johanna's husband is John's Schwager, but Mary's Schwippschwager.

Got it? In German, a Schwippschwager is a distant brother- or sister-in-law, that is, a sibling or spouse of those who've married into the family. The prefix Schwipp- could be an abbreviation of the word Geschwister, meaning siblings. In Austrian German, however, a Schwippschwager is known as a Schwiegerschwager.