In the context of the forums, the rootword of "necropost," which is "post," is most accurately defined as "a message published on a website." The prefix of the word, "necro-" most nearly means "dead." As such, the appropriate definition for a "necropost" (which can function as both a noun and a verb) would be "(v) the act of publishing a message on a website in a dead topic" or "(n) a message published on a website in a dead topic."

to post in a thread that has been "dead", by being inactive for a relatively long period.