to claim that your side of an argument is morally better than your opponents' side; to argue in a way that makes your side seem morally better
TAKE, CLAIM, SEIZE, ETC. THE MORAL HIGH GROUND
Meaning of TAKE, CLAIM, SEIZE, ETC. THE MORAL HIGH GROUND in English
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005