Scruple is a feeling of doubt or hesitation based on moral grounds, or a very small amount of something. (noun)
When you believe it is immoral to lie and you hesitate before lying because of this belief, this is an example of a time when you have scruples about lying.
When you only have a tiny piece of cake, this is an example of a portion which is a scruple.
See scruple in Webster's New World College Dictionary
noun
a very small quantity, amount, or part
an ancient Roman unit of weight equal to ounce
a unit of weight equal to dram apothecaries' weight or 20 grains (1.296 grams): abbrev. sc
a feeling of hesitancy, doubt, or uneasiness arising from difficulty in deciding what is right, proper, ethical, etc.; qualm or misgiving about something one thinks is wrong