'Jabangr' is an
abbreviation of the question: 'did you bang her?' It is used as an immediate response to friends of yours who are talking about various girls/women they've met and/or been on dates with.
It's not important whether or not you care about the person actually banging the girl.
Unlike the actual question--"did you bang her?"--which is intended to elicit an answer--the abbreviated phrase is used more as an
interjection designed to introduce the thought that maybe you should have thought harder about trying to bang the girl.
Paul: "So I met this girl
last night..."
Brett: "Jabangr."
Paul: "Ha, no we had dinner,
but then afterwards one of her friends
showed up, and..."
Brett: "Jabangr."
Paul: "Man, what a panoche you are."