I find this to be a word commonly understood but not easily defined. Many seem to use scruples to be synonymous with morals, and while that definition may make sense in some uses, the true definition of a scruple is a doubt. So while a scrupleless person may be immoral, it is because he never questions whether his actions are just, he has no doubt.
Thanks to John Kuhner for pointing out the origin of the word, which comes from Latin and means a little pebble. Cicero described doubt and anxiety as a little pebble caught in one’s shoe — a minor but nagging sensation telling you that something is wrong.