A person who jumps to conclusions and speaks or acts out without full knowledge of the current situation, and then winds up looking like a jackass when informed of what's really going on.
"The other day Dave had a case of Sudden Jackass Syndrome(S.J.A.S.) when he caught his girl on the phone talking all secretively. Then he started accusing her of cheating on him, asking who the guy was and if he was no longer good enough for her. And that is when she informed him that she was being all secretive is because she was planning his birthday party. I swear you could see him grow buckteeth, a tail, and some donkey ears!"