When someone gets so mad that it has the potential to ruin their whole day. Noticeable when someone lets their anger leak out, usually described in forms of yelling or raising their voice in defense.
When I told josh I screwed his girlfriend he maxed out.
"Dude are you serious? I can't believe you, out of all people, got me to max out."
"What?!?! I got an F on my Spanish exam? My parents are going to max out!!"
The act of going off program to potentially destroy your body with the excuse that its max out Monday by attempting; A max deadlift, max squat, max bench, or all of the above.