pinch hitter

noun

1. Baseball. a substitute who bats for a teammate, often at a critical moment of the game.
2. any substitute for another, especially in an emergency.

A substitute for another person, especially in an emergency. For example, Pat expected her mother to help with the baby, but just in case, she lined up her mother-in-law as pinch hitter. This expression comes from baseball, where it is used for a player substituting for another at bat at a critical point or in a tight situation (called a pinch since the late 1400s). [Late 1800s]