second reserve service; military reserve force of ex-servicemen
Historical military term referring to a reserve force composed of former soldiers, or the service in such a force. Not in common modern use.
In pre-war Japan, the second reserve was made up of soldiers who had left active service.
Compound of 後 (after, later) + 備 (prepare, equip) + 役 (duty, service). The exact historical derivation is uncertain; the term appears in pre-WWII Japanese military contexts.