The noun scruple, chiefly used in the plural scruples, denotes a feeling of doubt or hesitation with regard to the morality or propriety of a course of action.

Through French scrupule, this word is from Latin scrūpŭlus, denoting, literally, a small sharp or pointed stone.

Probably because such stones used to get into the open shoes of the Romans, Latin scrūpŭlus came to denote, figuratively, a pricking, uneasy sensation, hence trouble, doubt, scruple. For instance, in Pro Roscio Amerino (For Roscius of Ameria – 80 BC), the Roman orator, author and statesman Marcus Tullius Cicero (106-43 BC) declared:

Hunc sibi ex animo scrupulum, qui se dies noctesque stimulat ac pungit, ut evellatis, postulat, ut ad hanc suam prædam, tam nefariam, adjutores vos profiteamini.
     translation by C. D. Yonge (London, 1903):
He begs that you will take from his mind this uneasiness which day and night is pricking and harassing him, so as to profess yourselves his assistants in enjoying this his nefariously acquired booty.

Latin scrūpŭlus was a diminutive of scrūpus, denoting a rough or hard stone, that Cicero used figuratively for the usual scrūpŭlus in De re publica (On the Commonwealth – between 54 and 51 BC):

Ad haec illa dici solent primum ab iis, qui minime sunt in disserendo mali, qui in ea causa eo plus auctoritatis habent, quia, cum de viro bono quaeritur, quem apertum et simplicem volumus esse, non sunt in disputando vafri, non veteratores, non malitiosi; negant enim sapientem idcirco virum bonum esse, quod eum sua sponte ac per se bonitas et iustitia delectet, sed quod vacua metu, cura, sollicitudine, periculo vita bonorum virorum sit, contra autem improbis semper aliqui scrupus in animis haereat, semper iis ante oculos iudicia et supplicia versentur; nullum autem emolumentum esse, nullum iniustitia partum praemium tantum, semper ut timeas, semper ut adesse, semper ut impendere aliquam poenam putes, damna
     translation by C. D. Yonge (New York, 1877):
In reply to these statements, the following arguments are often adduced by those who are not unskilful in discussions, and who, in this question, have all the greater weight of authority, because, when we inquire, Who is a good man?—understanding by that term a frank and single-minded man—we have little need of captious casuists, quibblers, and slanderers. For those men assert that the wise man does not seek virtue because of the personal gratification which the practice of justice and beneficence procures him, but rather because the life of the good man is free from fear, care, solicitude, and peril; while, on the other hand, the wicked always feel in their souls a certain suspicion, and always behold before their eyes images of judgment and punishment. Do not you think, therefore, that there is any benefit, or that there is any advantage which can be procured by injustice, precious enough to counterbalance the constant pressure of remorse, and the haunting consciousness that retribution awaits the sinner, and hangs over his devoted head.

The noun scruple also denotes a unit of weight equal to 20 grains (⅓ drachm, 1/24 oz.), used by apothecaries; it is denoted by the character ℈.

It is from Latin scrūpŭlus, denoting a small unit of weight or measurement and commonly regarded as identical with scrūpŭlus, denoting a small stone, the sense small weight or measure being supposed to be developed from the etymological sense small stone—cf. the use of stone as a unit of weight.

However, because, in the sense of a small weight or measurement, the Latin word frequently appeared in forms with -i- such as scrīpulum, its origin might have been distinct from that of scrūpŭlus.

From the sense of a small unit of weight or measurement, scruple has been used figuratively in the sense of a very small quantity or amount, especially of a quality. At the beginning of Measure, For Measure (Folio 1, 1623), by the English poet and playwright William Shakespeare (1564-1616), Vincentio, the Duke of Vienna, who is about to leave the city, addresses Angelo, whom he appoints as his deputy:

There is a kinde of Character in thy life,
That to th’ obseruer, doth thy history
Fully vnfold: Thy selfe, and thy belongings
Are not thine owne so proper, as to waste
Thy selfe vpon thy vertues; they on thee:
Heauen doth with vs, as we, with Torches doe,
Not light them for themselues: For if our vertues
Did not goe forth of vs, ’twere all alike
As if we had them not: Spirits are not finely tonch’d [= touch’d],
But to fine issues: nor nature neuer lends
The smallest scruple of her excellence,
But like a thrifty goddesse, she determines
Her selfe the glory of a creditour,
Both thanks, and vse; but I do bend my speech
To one that can my part in him aduertise;
Hold therefore Angelo:
In our remoue, be thou at full, our selfe:
Mortallitie and Mercie in Vienna
Liue in thy tongue, and heart: Old Escalus
Though first in question, is thy secondary.
Take thy Commission.