The story
Morphologically, from Troyes, town in Northern France + grain, unit of mass.
Etymology from Wiktionary, CC BY-SA 4.0.
troy grain
Today it means: A unit of mass, equal to 1/24 of a pennyweight or 1/480 of a troy ounce, fixed at 0.064 798 91 grams under the metric system. Full definition →