Prepared and waiting to act. Readiness is a state that costs something to hold.
Old English geræde (prepared, equipped for riding), from Proto-Germanic garaidijaz; to be ready was first to be equipped for a specific undertaking, and the sense of general preparedness followed from that narrower one.
Prepared and waiting to act. The root's equipping for a specific task is the honest version: readiness is not a resting state, it is a held state, and holding it costs something.