distinction between right and wrong
Almost exclusively used in the negative phrase 頑是ない (or 頑是がない), meaning 'innocent' or 'unable to tell right from wrong', especially of young children. The standalone noun is rare.
He's still a child who can't tell right from wrong, so he didn't mean any harm.
innocent; helpless
The exact derivation is uncertain; 頑 (stubborn) and 是 (right/correct) combine to suggest a stubborn or fixed sense of what is right, but the word is now fossilized in the negative form.