So a person who acts like a know-it-all might be a megalomaniac, or a narcissist, or someone overcompensating for depression, but it could just be that they have a very common disorder: Banal human arrogance.
Studies have found that quite a lot of people have distorted views of themselves. There’s the Lake Woebegone effect, where everyone thinks they’re above average. There’s the , wherein the most incompetent people think of themselves as the most competent, exactly because the first thing anyone learns in any field is how little they know and how much it matters to learn what they don’t know.
So if a behavior trait is omnipresent and actually not all that maladaptive (just annoying), it’s not a disorder.