When you're so angery that you could be considered a possible homocide offender.
Comes from the saying: Jack's got the key to the gun case. Wanna see?
I was in Jack mode when Daniel's project got a higher grade than mine.
To go into a stealing frenzy. From the word jack (as in to steal). Literally "stealing mode". Often yelled as one steals something in a very obvious manner.
"Jack Mode!" *runs off with object*