What does " three inches of plate in my skull" mean?

What does “three inches of plate in my skull” mean? I tried to google it, but it seems it's not a set expression.

“According to my archive I was constantly in some fight or another over email. I apparently have three inches of plate in my skull. And in fact, because I believed, and have believed for so long, that I once was passive but am no longer, I think I tend to be even more likely to be passive-aggressively aggrieved than the typical person.”

Solution 1:

"Plate" refers to the type of metal plate used to repair skull damage that someone may have suffered at the hands of an attacker with a bat or other solid object.

The implication is that the writer has had a large number of fights, some of which were life-threatening to such a degree that a fractured skull had to be repaired at least once.

Also implied is a degree of invulnerability to pain now, and a predilection towards future violence. Or at least no avoidance of fights. As @Rob_Ster commented:

the three inches seems more appropriate to the armor on a warship than to a medical prosthesis. There's word-play afoot in the cited text.