Scrupulum, meaning a tiny stone (from scrupus sharp stone), indicates a weight of 1/24 of an ounce or, by extension, of other measures. Metaphorically, the stone is thought to be sharp and pricking, like a thorn.
The forms scripulum, scriptulum, scriplus and scriptulus can be found; scriptulum being also associated with the lines on a draughtboard.
Félix Gaffiot: Dictionnaire Illustré Latin Français, Hachette 1934