definition 1: a moral or ethical consideration or doubt that gives rise to uneasiness, hesitation, or restraint; qualm.
example: The man had no scruples and didn’t think twice before swindling the old couple out of their savings.
definition 2: a unit of apothecaries‘ weight equal to twenty grains or 1.296 grams.
definition 3: a tiny part or quantity.
definition: to refrain for ethical or moral reasons; have scruples.
example: Norris had done absolutely nothing, which, even by implication, could be construed into a dereliction of duty; but it was sufficient that he was hated by Leicester, who had not scrupled, over and over again, to denounce this first general of England as a fool, a coward, a knave, and a liar. (John Lothrop Motley – History of the United Netherlands from the Death of William the Silent to the Twelve Year’s Truce — Complete (1584-1609))
The most frequent noun sense of “scruple,” definition 1, appears much more often in the plural than in the singular form. “Unlike that shark, I have scruples!” “I lacked his scruples, and stuffed as much of the money in my backpack as would fit.” However, It is by no means incorrect to use “scruple” in the singular:
Though they regarded each other with mortal aversion, though neither of them would have had any scruple about persecuting the other, they had much in common.
He felt a scruple against bringing a profane instrument into sacred space.
And although perhaps more frequent in the past in phrases such as “without scruple,” and “made no scruple,” it is still acceptable to use “scruple” as an uncountable noun, as if it were a substitute for “conscience” or “opposition”: She displayed a powerful mix of religious scruple, prudery and racial pride.
The plural form, “scruples,” can be used without elaboration, “He has no scruples,” in the same way that one can say “He lacks morals.” These are statements about the character of someone, rather than about a particular doubt one has about committing a particular act.