Justin Bieber has gone on the attack against the former neighbor who is suing him over the 2014 egging incident, saying the neighbor claiming emotional distress is actually deep into the porn business.

As we've reported for years now, Jeffrey Schwartz and his wife are suing Bieber for what they say is severe emotional distress from Bieber's hijinks when he lived in the Calabasas neighborhood ... spitting, fighting and egging to be specific. Schwartz already got $80k to repair the damage to his home.

Now Bieber is firing back, saying if Jeffrey Schwartz has emotional distress, it could be because he's hiding something from his family. Bieber claims in new legal docs Schwartz bankrolled a pornographic company called Erotizen. Schwartz denied he kept the company from his family.

And Bieber says there are other reasons Schwartz may be suffering emotional distress ... claiming his company, Seller Networks, was facing countless complaints from customers for promising to sell their cars within a certain time and failing to deliver.