What is a “scruple” a measurement of?

A scruple is a unit of weight in the apothecaries’ system, equal to 20 grains, or one-third dram, and equivalent to 1.296 grams. The apothecaries' system or apothecaries' weights and measures is a historical system of mass and volume units that were used by physicians and apothecaries for medical recipes, and also sometimes by scientists.

In ancient times, when coinage weights customarily furnished the lower subdivisions of weight systems, the scruple (from Latin scrupulus, “small stone” or “pebble”) was a unit of Roman commercial weight as well as a unit of coinage weight. One drachma, the basic Greek silver unit, consisted of three scruples.

A grain is a unit of weight equal to 0.065 gram. A dram, unit of weight in the apothecaries’ system contains 3 scruples (3.888 grams).