"Cross-posting" is the act of asking a question of more than one site within a system.
As far as I know, it has always had a poor reputation, because doing so encourages answers that are not seen among the various sites. (I'm sure there are many reasons not to like cross-posting, but that's not the issue here)
Also, as far as I know, it is a behavior that is often desired by question-askers, especially if the question is ambiguous or broad or touches on disparate areas (e.g. "What is the words for the dial on the dashboard of a glider?" (good for ELU or Aviation) or "What is a good calculation method for attenuation of the radiosignal from Voyager I?" (good for math, physics, space exploration, scientificcomputing.SE) ). Which is all to say that it is a well worn concept well deserving of a name even if there is no specific mechanism that implements it.
I don't know the full history of it, but I became aware of the term with usenet in the late 90's where there was a mechanism to author a single message and have it appear on multiple usenet sites at once.
My question is whether 'cross-posting' is figurative. Does it require that the particular system being specifically engineer an option to post to more than one site (like usenet)? Or can it be used, literally, even if done by hand? Or does it mean posting to entirely separate engineered mechanisms like SE, Yahoo Answers, and my grandmother's blog by hand, just all in the same time frame? Which of these latter does the word apply to (or others if not these)?