on the bench

Short definition from Wiktionary

Prepositional phrase

1. 1
   Used other than figuratively or idiomatically: see on, the, bench.
2. 2
   Presiding over a court.
3. 3
   Not playing in an ongoing game, either waiting to replace an active player or removed from competition by injury, penalty, etc.
4. 4
   Waiting to command his first vessel and assume the position of post-captain, ensuring further promotion will remain a matter of seniority rather than merit or ability.