We all know what having scruples means. The etymology of "scruples" is from a Latin word, scrupulus, meaning "small unit of measurement" and scrupus, meaning a rough stone.

Does anyone know how it evolved into meaning "an uneasy feeling arising from conscience or principles?"

The distinction is in Latin, and they were borrowed separately. Latin had a word scrupulus 'pebble, small sharp stone', which could also be used figuratively for 'nagging doubt, uneasiness'. This sense also gave them an adjective scrupulosus meaning both 'stony, rough' and 'precise'.

They also had scripulum, scrupulum, scrupulus 'a unit of weight', presumably the same word and in origin meaning 'pebble'. The change of vowel indicates that it's an old word, as /i/, /u/ fluctuated near labials in Old Latin, as in optumos, optimos.

The OED records the sense of doubt earlier (1526) than the apothecaries' weight (1564) in English.

My logophile friend sent me this note about 'scruple':

"Scruple comes from the Latin for a rough pebble. So it also means a nagging pebble in the shoe of our conscience."

Now, I like that analogy.

So is there a "standard pebble"? I seem to recall that apothecary measure includes not just grains and drams but also scruples...