‘Dead nuts’ is an idiom used by skilled workers or machinists to refer to something accurate—not just ‘good enough’ accurate but precise, dead center, right on the spot accurate. The word ‘dead’ here could be connected to ‘dead on,’ which means ‘exact’ or ‘exactly right.’

Adjectival use, typically describing enthusiasm about or fondness for someone or something came about in the late 18th century. In Britain, the term was often used in the phrase "dead nuts on," as "She is dead nuts on the boy next door."