/Jak/./En/./Tha/./Baks/ adj. 1. To drop in on someone completely unannounced thus upsetting them or potentially ruining something. 2. Dropping in without calling. 3. Showing up unannounced and fucking everything up.
1. He was about to get some from girl on the side but his woman Jack in the boxed his ass.
2. I was sleeping like a baby till I got jack in the boxed by that douche Bobby.