A bit of a combination of origins. "dead" as in "dead naught" or "dead plumb" has roots in nautical measurement and carpentry to mean "precisely". In this case "nuts" refers to the double zero "00" common on measurement and surveying equipment (non-electronic). If a measurement is precisely at the double zero, such as when closing out a surveying loop, this is the most precision possible with the given piece of equipment. You may also hear surveyors refer to this as "balls on" measurement for the same reason.
