con-sci-en-tious / kŏn-shē-ĕn-shəs
1. influenced by conscience or principals
An intelligent and conscientious opposition is a part of loyalty to country.
Bainbridge Colby, 1869 – 1950
2. demonstrating great care and attention to details; diligent; careful
He who has no taste for order, will be often wrong in his judgment, and seldom considerate or conscientious in his actions.
Johann Kaspar Lavater, 1741 – 1801
From a combination of the Latin noun conscientia (consciousness, cognizance, moral sense) and the Latin adjectival suffix -osus (full of).
Thank you to Allen Ward for providing this etymology.