To compare two people to Frick and Frack implies they have a close relation and are very similar in many ways. "Stalin and Hitler go together like Frick and Frack."

The origin comes from a pair of Swiss ice skaters who performed comedy stunts for the Ice Follies in the late '30s. Each dressed in matching lederhosen and wowed the crowds with a trick they called the "cantilever spread-eagle". Frick's real name was Werner Groebli and Frack's was Hans Mauche.

For a time Frick and Frack were so popular they were the highest paid professional ice skaters in the world. Their popularity and association was so well known that they became synonymous with interpersonal symmetry, much like how people might say "they go together like Gilbert and Sullivan or Lennon/McCartney."