Green’s Dictionary of Slang

1. to initiate someone into a street gang.

1985		T.R. Houser Central Sl. 69: jumped-in A session where one who desires entry into a gang is beaten up by the other gang members as his or her initiation.
1996		P. Beatty White Boy Shuffle 57: Maybe this was one of those jumping-in rituals I’d seen on the PBS documenatries entitled Our Youth at Risk.
2011		F. Bill ‘Crimes in Southern Indiana’ in Crimes in Southern Indiana [ebook] [A]fter the jumping-in he had to seal his initiation, spill someone else’s blood.

2. in passive, to be initiated into a street gang.

1991		L. Bing Do or Die (1992) 20: My cousins took me up the park one day, see, and we got drunk and I got hit and I fought back. That’s how I got jumped in – the old fashioned way.
1997		G. Sikes 8 Ball Chicks (1998) 22: If she refused to be ‘jumped in,’ allowing the PlayGirls to punch and kick her in a ritual pummeling, she would lose respect.
2000		W. Shaw Westsiders 32: According to the mythology of gang-banging, new members are ‘jumped in’ — that is, beaten viciously by other members.
2001		(con. 1990s) in J. Miller One of the Guys 48: ‘One day I just woke up and I just say I wanna be one of them [i.e. the gang], and then I told them and they they jumped me in the ’hood’.
2009		J. Stahl Pain Killers 48: Inmate jumped into La Eme 11/06.