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1.
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.

See jack
"Jack Mode!" *runs off with object*
by n00bdragon December 07, 2004
 
2.
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.
by darth_eowyn September 27, 2006