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1.
A guy who is or was your bro/broski but did something to make him nosedive down your broski depth chart to the point where he still might be your bro/broski but you treat him like a jabroni and don't want shit all to do with him either permanently or temporarily because he might be a full on bitch and you don't want to deal with his bullshit.

A jabroski only gets few chances to work his way back up to broski status, unless the guy is keeping the jabroski in his back pocket because he's using him for something whether it be his money, alcohol, drugs, fame, connections, women, crib, ride or any other bullshit he arrogantly and selfishly deems useful to himself and wants to continue to take advantage of.
Example 1

Bro 1: Is he fuckin' serious bro?

Bro 2: I think he is bro.

Bro 3: Sup bro? Near beer's the shit kid.

Bro 1: Don't kid me bro. Walk away....I said walk away in the other direction. Nah don't look over here, I said no eye contact, keep walking jabroski. Go find ya balls.

Bro 2: You believe this fuckin' guy.

Bro 1: He ain't on our level bro, lets go find some quality girls that's DTF in the washroom over here. No grenades tonight aight bro?

Example 2

Some Dude on Twitter: But last week I was broski of the week Zack. What happened to us?

Zack Ryder: Yeah but this week you didn't buy the Zack Ryder t-shirt and like me on Facebook like I told you to jabroski. Now take care, spike your hair. Woo woo woo, you know it.
by The Mooski August 12, 2011
 
2.
A jabroni who's still a bro.

Someone who's all talk but none the less is still a friend and will most likely get invited to hang again.
Rich: Marty's full of himself. He always talks smack but never lives up to it.

Derek: Yeah but it wouldn't be the same without him around, and it would be awkward to invite everyone but him next time.

Rich: What a jabroski.
by Murphy Burns January 03, 2011