Archaic expression, used in many trades to mean 'spot on', or absolutely accurate. It originates from the times when machinsts machines were
set with a set of nuts, on a fine threaded screw inside the machine which controled it's movements to and froe. The
nut would be set at absolute zero, or the stop, or limit of the machine. When the
nut was tightened, and the zero set, it would be considered a dead
nut.