He's always relaxed and presses his stomach and chest out towards everyone, clearly not intimidated even a little bit by anyone, and doesn't view anyone as a threat.
Our primal instinct demands that when we feel threatened, we guard our vitals like our stomach and chest, but Negan not only doesn't do that, he does the exact opposite, he leaves his vitals wide open to everyone as if to say "go ahead, try it".
And he at one point challenged Rick to grab the axe and drive it into Negan's brain, he concluded, "see how you do, grab the axe. GRAB THE DAMN AXE!".
When Carl hid in one of Negan's vans and shot one of Negan's men, before demanding that Negan show himself, Negan whistled and once again presses his body out, trying to look confident.
He tells Carl that he's adorable, as one man with a rifle couldn't possibly threaten an entire compound of armed men. But then Negan shows the truth by, for the first time, hiding his vitals when he hides behind one of his men.
Negan then said "Kid, I'm not gonna lie, you scare the SH! T outta me". Despite his clear attempt to sound sarcastic, I can tell that he really meant it. Carl was willing to facedown everyone to avenge his friends, and such resolve and audacity was enough to scare Negan.
We do most of our talking through body language, usually without even realizing it. Negan wanted to remain tough and strong in front of his men, but him hiding his vitals behind one of his men told the truth, Negan really IS scared of Carl

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