An old video of XXX is making the rounds online, which shows him dancing behind a girl filming herself ... and then hitting her on the side of her head before the camera cuts out.

Law enforcement sources tell TMZ, prosecutors in XXX's domestic violence case are reviewing the video -- which appears to be old based on his hair style. It's unclear how it might affect his case in light of a judge recently letting him out of the house to tour.

As for XXX ... his attorney, Jaclyn Broudy, tells us it's clear from the video that her client and the girl are friends and "the video was taken completely in jest." She also points out the footage is dated ... and that XXX is a changed person, even though some people are still trying to harm him by releasing an old video that she says shows nothing.