comfortable as an old shoe, as

Familiar and very much at ease, as in Kathy's really enjoying her third summer at the same camp—for her it's comfortable as an old shoe. This simile was once put as easy as an old shoe, first recorded in J. T. Brockett's North Country Glossary (1825). It was most often applied to friends, as in the proverb Old shoes are easy, old friends are best, but today it is used more broadly for easy familiarity in any situation.