backfill
n.— «I was mobilized to a stateside assignment at Fort Eustis in Virginia, where I would administer anesthesia at the McDonald Army Community Hospital. I was known as “backfill,” a term for identifying someone who fills the void left by a soldier being deployed overseas.» —“Destination: Iraq” Rutland Herald (Vermont) Oct. 22, 2006. (source: Double-Tongued Dictionary)