A lengthy contraction. Use it your own way, who knows what it actually means. It’s too rusty to translate. The word can be found on Reddit, Tumblr, and probably more, most likely originating on social media.
Mike: who'd'ven't'f'I'dn't've'nt'f'I'd'ven't'f'Ihavn'tn't'd'I've-
John: Mike, shut up, we’re in an interview
When someone says "Can I Vent?" DON'T MAKE FUN OF THEM, They Are asking permission to share their feelings and what they're going through. If you REALLY WANT TO HEAR IT, But knowing that you don't have time you can easily say the truth on why you can't comfort them while they vent.