C'ha sempre da ridì
When someone has to have the last word in any discussion, no matter the subject, even if they are not involved in the conversation in the first place.
Example 1:
Person A: It was raining all day yesterday, I did not go anywhere.
Person B: Yeah, I was outside and got soaked.
Random person: Well, it actually did not rain between 3 and 4 pm
Person A: C'ha sempre da ridì!

Person B: C'ha, c'ha *Making shotgun noise*, sempre da ridì.

Example 2:
Person A: I wonder how taste works.
Person B: I believe that the tongue is divided into areas, each dedicated to a specific taste.
Person CSDR: Well, *pushes glasses back with his finger* that theory was debunked in 2006 with the general rule being that there are five basic tastes identified so far, and the entire tongue can sense all of these tastes more or less equally. Blah blah blah... science facts.. shit no one cares about.. blah blah blah...
Person A&B: C'ha sempre da ridì! *screams and laughter ensues*