Something posted to multiple sites. This can be an image, video, message, etc, as long as it's posted by the same person on more than one website, it's a crosspost.
Person 1: Why did you put that meme on instagram and twitter?
Person 2: I like to crosspost.
I have no friggin' idea. Some old bag once told me that if I didn't slow down in her parking lot, she would take the crosspiece out of my ass. So define it how you like.
He went to kick the dude in the nuts but instead broke his crosspiece.