Nothing to do with boats at all, although I am sure there has got to be a piece of wood on a traditional wood hulled boat or sailboat called a cross post.
Crossposting is posting the same message across multiple social media at the same time. The typical example is twitter, facebook, youtube, instagram. All will have the message according to what they can display/handle.
A message that is crossposted is just that.
A lot of blog/forum frequent users will use the term in the post heading to let other users on the forum know they are soliciting the info on other sites. It also shows up on sites like reddit and may be a sign that a post is in multiple threads.
We're pretty simple here. Ya just gotta ask yer question and we'll all give ya answers, and ya takes yer chances.