He is wise in his own conceit, always thinking he knows better than everyone else.
Он мудрый в собственных глазах, всегда думает, что знает лучше всех.
Definitions
To be 'wise in (someone's) own conceit' means to consider oneself very intelligent, knowledgeable, or clever, often in an arrogant or self-important way, regardless of whether this self-assessment is accurate.
