con- ‘with’ + scire ‘know’

source: do it angel GIF by Jin - Find & Share on GIPHY

an inner feeling or voice viewed as acting as a guide to the rightness or wrongness of one's behavior:

良心, 道義心, 善悪の判断力; 良心にしたがうこと; 後ろめたさ (!個人の良心を表す場合はa ～/one's ～(es); その際しばしば修飾語を伴う)

e.g. he had a guilty conscience about his desires

e.g. Ben was suffering a pang of conscience.

by any reasonable standard; by all that is fair: they have in conscience done all they could.

weighing heavily and guiltily on one's mind: an act of providence had prevented him from having a death on his conscience.

conscienceless | ˈkänSH(ə)nsləs | adjective

Middle English (also in the sense ‘inner thoughts or knowledge’): via Old French from Latin conscientia, from conscient- ‘being privy to’, from the verb conscire, from con- ‘with’ + scire ‘know’.